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_gstat/src/gstat.rs

@@ -16,10 +16,6 @@ impl GStat {
         Default::default()
     }
 
-    pub fn usage() -> &'static str {
-        "Usage: gstat"
-    }
-
     pub fn gstat(
         &self,
         value: &Value,

crates/nu_plugin_gstat/src/nu/mod.rs

@@ -21,7 +21,7 @@ impl SimplePluginCommand for GStat {
         "gstat"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the git status of a repo"
     }