Change the usage misnomer to "description" (#13598)

# Description
    
The meaning of the word usage is specific to describing how a command
function is *used* and not a synonym for general description. Usage can
be used to describe the SYNOPSIS or EXAMPLES sections of a man page
where the permitted argument combinations are shown or example *uses*
are given.
Let's not confuse people and call it what it is a description.

Our `help` command already creates its own *Usage* section based on the
available arguments and doesn't refer to the description with usage.

# User-Facing Changes

`help commands` and `scope commands` will now use `description` or
`extra_description`
`usage`-> `description`
`extra_usage` -> `extra_description`

Breaking change in the plugin protocol:

In the signature record communicated with the engine.
`usage`-> `description`
`extra_usage` -> `extra_description`

The same rename also takes place for the methods on
`SimplePluginCommand` and `PluginCommand`

# Tests + Formatting
- Updated plugin protocol specific changes
# After Submitting
- [ ] update plugin protocol doc

crates/nu_plugin_example/src/commands/call_decl.rs

@@ -33,11 +33,11 @@ impl PluginCommand for CallDecl {
             )
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Demonstrates calling other commands from plugins using `call_decl()`."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "
 The arguments will not be typechecked at parse time. This command is for
 demonstration only, and should not be used for anything real.

crates/nu_plugin_example/src/commands/collect_bytes.rs

@@ -16,7 +16,7 @@ impl PluginCommand for CollectBytes {
         "example collect-bytes"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example transformer to byte stream"
     }
 

crates/nu_plugin_example/src/commands/config.rs

@@ -39,11 +39,11 @@ impl SimplePluginCommand for Config {
         "example config"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Show plugin configuration"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "The configuration is set under $env.config.plugins.example"
     }
 

crates/nu_plugin_example/src/commands/ctrlc.rs

@@ -15,7 +15,7 @@ impl PluginCommand for Ctrlc {
         "example ctrlc"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example command that demonstrates registering an interrupt signal handler"
     }
 

crates/nu_plugin_example/src/commands/disable_gc.rs

@@ -12,11 +12,11 @@ impl SimplePluginCommand for DisableGc {
         "example disable-gc"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Disable the plugin garbage collector for `example`"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "\
 Plugins are garbage collected by default after a period of inactivity. This
 behavior is configurable with `$env.config.plugin_gc.default`, or to change it

crates/nu_plugin_example/src/commands/echo.rs

@@ -13,7 +13,7 @@ impl PluginCommand for Echo {
         "example echo"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example stream consumer that outputs the received input"
     }
 

crates/nu_plugin_example/src/commands/env.rs

@@ -12,11 +12,11 @@ impl SimplePluginCommand for Env {
         "example env"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get environment variable(s)"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Returns all environment variables if no name provided"
     }
 

crates/nu_plugin_example/src/commands/for_each.rs

@@ -13,11 +13,11 @@ impl PluginCommand for ForEach {
         "example for-each"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example execution of a closure with a stream"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Prints each value the closure returns to stderr"
     }
 

crates/nu_plugin_example/src/commands/generate.rs

@@ -16,11 +16,11 @@ impl PluginCommand for Generate {
         "example generate"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example execution of a closure to produce a stream"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "See the builtin `generate` command"
     }
 

crates/nu_plugin_example/src/commands/main.rs

@@ -12,11 +12,11 @@ impl SimplePluginCommand for Main {
         "example"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example commands for Nushell plugins"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"
 The `example` plugin demonstrates usage of the Nushell plugin API.
 

crates/nu_plugin_example/src/commands/one.rs

@@ -12,12 +12,12 @@ impl SimplePluginCommand for One {
         "example one"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Plugin test example 1. Returns Value::Nothing"
     }
 
-    fn extra_usage(&self) -> &str {
-        "Extra usage for example one"
+    fn extra_description(&self) -> &str {
+        "Extra description for example one"
     }
 
     fn signature(&self) -> Signature {

crates/nu_plugin_example/src/commands/seq.rs

@@ -16,7 +16,7 @@ impl PluginCommand for Seq {
         "example seq"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example stream generator for a list of values"
     }
 

crates/nu_plugin_example/src/commands/sum.rs

@@ -13,7 +13,7 @@ impl PluginCommand for Sum {
         "example sum"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example stream consumer for a list of values"
     }
 

crates/nu_plugin_example/src/commands/three.rs

@@ -12,7 +12,7 @@ impl SimplePluginCommand for Three {
         "example three"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Plugin test example 3. Returns labeled error"
     }
 

crates/nu_plugin_example/src/commands/two.rs

@@ -12,7 +12,7 @@ impl SimplePluginCommand for Two {
         "example two"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Plugin test example 2. Returns list of records"
     }
 

crates/nu_plugin_example/src/commands/view_span.rs

@@ -13,11 +13,11 @@ impl SimplePluginCommand for ViewSpan {
         "example view span"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Example command for looking up the contents of a parser span"
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Shows the original source code of the expression that generated the value passed as input."
     }