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-command/src/debug/ast.rs

@@ -10,7 +10,7 @@ impl Command for Ast {
         "ast"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Print the abstract syntax tree (ast) for a pipeline."
     }
 

crates/nu-command/src/debug/debug_.rs

@@ -8,7 +8,7 @@ impl Command for Debug {
         "debug"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Debug print the value(s) piped in."
     }
 

crates/nu-command/src/debug/explain.rs

@@ -12,7 +12,7 @@ impl Command for Explain {
         "explain"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Explain closure contents."
     }
 

crates/nu-command/src/debug/info.rs

@@ -20,11 +20,11 @@ impl Command for DebugInfo {
         "debug info"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View process memory info."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "This command is meant for debugging purposes.\nIt shows you the process information and system memory information."
     }
 

crates/nu-command/src/debug/inspect.rs

@@ -10,7 +10,7 @@ impl Command for Inspect {
         "inspect"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Inspect pipeline results while running a pipeline."
     }
 

crates/nu-command/src/debug/metadata.rs

@@ -12,7 +12,7 @@ impl Command for Metadata {
         "metadata"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the metadata for items in the stream."
     }
 

crates/nu-command/src/debug/metadata_set.rs

@@ -9,7 +9,7 @@ impl Command for MetadataSet {
         "metadata set"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Set the metadata for items in the stream."
     }
 

crates/nu-command/src/debug/profile.rs

@@ -43,11 +43,11 @@ impl Command for DebugProfile {
             .category(Category::Debug)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Profile pipeline elements in a closure."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"The profiler profiles every evaluated pipeline element inside a closure, stepping into all
 commands calls and other blocks/closures.
 

crates/nu-command/src/debug/timeit.rs

@@ -9,7 +9,7 @@ impl Command for TimeIt {
         "timeit"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Time the running time of a block."
     }
 

crates/nu-command/src/debug/view.rs

@@ -14,11 +14,11 @@ impl Command for View {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Various commands for viewing debug information."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "You must use one of the following subcommands. Using this command as-is will only produce this help message."
     }
 

crates/nu-command/src/debug/view_files.rs

@@ -8,11 +8,11 @@ impl Command for ViewFiles {
         "view files"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View the files registered in nushell's EngineState memory."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "These are files parsed and loaded at runtime."
     }
 

crates/nu-command/src/debug/view_ir.rs

@@ -29,11 +29,11 @@ impl Command for ViewIr {
             .category(Category::Debug)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View the compiled IR code for a block of code."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "
 The target can be a closure, the name of a custom command, or an internal block
 ID. Closure literals within IR dumps often reference the block by ID (e.g.

crates/nu-command/src/debug/view_source.rs

@@ -11,7 +11,7 @@ impl Command for ViewSource {
         "view source"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View a block, module, or a definition."
     }
 

crates/nu-command/src/debug/view_span.rs

@@ -8,11 +8,11 @@ impl Command for ViewSpan {
         "view span"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View the contents of a span."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "This command is meant for debugging purposes.\nIt allows you to view the contents of nushell spans.\nOne way to get spans is to pipe something into 'debug --raw'.\nThen you can use the Span { start, end } values as the start and end values for this command."
     }