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/network/url/build_query.rs

@@ -17,7 +17,7 @@ impl Command for SubCommand {
             .category(Category::Network)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts record or table into query string applying percent-encoding."
     }
 

crates/nu-command/src/network/url/decode.rs

@@ -31,7 +31,7 @@ impl Command for SubCommand {
             .category(Category::Strings)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts a percent-encoded web safe string to a string."
     }
 

crates/nu-command/src/network/url/encode.rs

@@ -32,7 +32,7 @@ impl Command for SubCommand {
             .category(Category::Strings)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts a string to a percent encoded web safe string."
     }
 

crates/nu-command/src/network/url/join.rs

@@ -14,7 +14,7 @@ impl Command for SubCommand {
             .category(Category::Network)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts a record to url."
     }
 

crates/nu-command/src/network/url/parse.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
             .category(Category::Network)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Parses a url."
     }
 

crates/nu-command/src/network/url/url_.rs

@@ -14,11 +14,11 @@ impl Command for Url {
             .category(Category::Network)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Various commands for working with URLs."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "You must use one of the following subcommands. Using this command as-is will only produce this help message."
     }