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/env/config/config_.rs

@@ -14,11 +14,11 @@ impl Command for ConfigMeta {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Edit nushell configuration files."
     }
 
-    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/env/config/config_env.rs

@@ -19,7 +19,7 @@ impl Command for ConfigEnv {
         // TODO: Signature narrower than what run actually supports theoretically
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Edit nu environment configurations."
     }
 

crates/nu-command/src/env/config/config_nu.rs

@@ -23,7 +23,7 @@ impl Command for ConfigNu {
         // TODO: Signature narrower than what run actually supports theoretically
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Edit nu configurations."
     }
 

crates/nu-command/src/env/config/config_reset.rs

@@ -22,7 +22,7 @@ impl Command for ConfigReset {
             .category(Category::Env)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Reset nushell environment configurations to default, and saves old config files in the config location as oldconfig.nu and oldenv.nu."
     }
 

crates/nu-command/src/env/export_env.rs

@@ -20,11 +20,11 @@ impl Command for ExportEnv {
             .category(Category::Env)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Run a block and preserve its environment in a current scope."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"This command is a parser keyword. For details, check:
   https://www.nushell.sh/book/thinking_in_nu.html"#
     }

crates/nu-command/src/env/load_env.rs

@@ -8,7 +8,7 @@ impl Command for LoadEnv {
         "load-env"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Loads an environment update from a record."
     }
 

crates/nu-command/src/env/source_env.rs

@@ -25,11 +25,11 @@ impl Command for SourceEnv {
             .category(Category::Core)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Source the environment from a source file into the current environment."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"This command is a parser keyword. For details, check:
   https://www.nushell.sh/book/thinking_in_nu.html"#
     }

crates/nu-command/src/env/with_env.rs

@@ -25,7 +25,7 @@ impl Command for WithEnv {
             .category(Category::Env)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Runs a block with an environment variable set."
     }