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

src/command.rs

@@ -304,7 +304,7 @@ impl Command for Nu {
 
     fn signature(&self) -> Signature {
         let mut signature = Signature::build("nu")
-            .usage("The nushell language and shell.")
+            .description("The nushell language and shell.")
             .named(
                 "commands",
                 SyntaxShape::String,
@@ -467,7 +467,7 @@ impl Command for Nu {
         signature
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "The nushell language and shell."
     }