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-core/src/serializers/tests.rs

@@ -254,10 +254,13 @@ macro_rules! generate_tests {
                 ) => {
                     assert_eq!(returned_signature.len(), 1);
                     assert_eq!(signature.sig.name, returned_signature[0].sig.name);
-                    assert_eq!(signature.sig.usage, returned_signature[0].sig.usage);
                     assert_eq!(
-                        signature.sig.extra_usage,
-                        returned_signature[0].sig.extra_usage
+                        signature.sig.description,
+                        returned_signature[0].sig.description
+                    );
+                    assert_eq!(
+                        signature.sig.extra_description,
+                        returned_signature[0].sig.extra_description
                     );
                     assert_eq!(signature.sig.is_filter, returned_signature[0].sig.is_filter);