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-command/src/database/commands/into_sqlite.rs

@@ -46,7 +46,7 @@ impl Command for IntoSqliteDb {
         operate(engine_state, stack, call, input)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert table into a SQLite database."
     }
 

crates/nu-command/src/database/commands/query_db.rs

@@ -27,7 +27,7 @@ impl Command for QueryDb {
             .category(Category::Database)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Query a database using SQL."
     }
 

crates/nu-command/src/database/commands/schema.rs

@@ -17,7 +17,7 @@ impl Command for SchemaDb {
             .category(Category::Database)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Show the schema of a SQLite database."
     }