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-engine/src/declaration.rs

@@ -32,12 +32,12 @@ impl Command for PluginDeclaration {
         self.signature.sig.clone()
     }
 
-    fn usage(&self) -> &str {
-        self.signature.sig.usage.as_str()
+    fn description(&self) -> &str {
+        self.signature.sig.description.as_str()
     }
 
-    fn extra_usage(&self) -> &str {
-        self.signature.sig.extra_usage.as_str()
+    fn extra_description(&self) -> &str {
+        self.signature.sig.extra_description.as_str()
     }
 
     fn search_terms(&self) -> Vec<&str> {