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-cmd-plugin/src/commands/plugin/add.rs

@@ -37,11 +37,11 @@ impl Command for PluginAdd {
             .category(Category::Plugin)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Add a plugin to the plugin registry file."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"
 This does not load the plugin commands into the scope - see `plugin use` for
 that.

crates/nu-cmd-plugin/src/commands/plugin/list.rs

@@ -29,7 +29,7 @@ impl Command for PluginList {
             .category(Category::Plugin)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "List installed plugins."
     }
 

crates/nu-cmd-plugin/src/commands/plugin/mod.rs

@@ -26,7 +26,7 @@ impl Command for PluginCommand {
             .category(Category::Plugin)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Commands for managing plugins."
     }
 

crates/nu-cmd-plugin/src/commands/plugin/rm.rs

@@ -33,11 +33,11 @@ impl Command for PluginRm {
             .category(Category::Plugin)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Remove a plugin from the plugin registry file."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"
 This does not remove the plugin commands from the current scope or from `plugin
 list` in the current shell. It instead removes the plugin from the plugin

crates/nu-cmd-plugin/src/commands/plugin/stop.rs

@@ -21,7 +21,7 @@ impl Command for PluginStop {
             .category(Category::Plugin)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Stop an installed plugin if it was running."
     }
 

crates/nu-cmd-plugin/src/commands/plugin/use_.rs

@@ -9,7 +9,7 @@ impl Command for PluginUse {
         "plugin use"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Load a plugin from the plugin registry file into scope."
     }
 
@@ -30,7 +30,7 @@ impl Command for PluginUse {
             .category(Category::Plugin)
     }
 
-    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