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_python/nu_plugin_python_example.py

@@ -44,8 +44,8 @@ def signatures():
             {
                 "sig": {
                     "name": "nu-python",
-                    "usage": "Signature test for Python",
-                    "extra_usage": "",
+                    "description": "Signature test for Python",
+                    "extra_description": "",
                     "required_positional": [
                         {
                             "name": "a",