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_polars/src/cache/get.rs

@@ -21,7 +21,7 @@ impl PluginCommand for CacheGet {
         "polars store-get"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Gets a Dataframe or other object from the plugin cache."
     }
 

crates/nu_plugin_polars/src/cache/list.rs

@@ -15,7 +15,7 @@ impl PluginCommand for ListDF {
         "polars store-ls"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Lists stored polars objects."
     }
 

crates/nu_plugin_polars/src/cache/rm.rs

@@ -17,7 +17,7 @@ impl PluginCommand for CacheRemove {
         "polars store-rm"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Removes a stored Dataframe or other object from the plugin cache."
     }