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/path/basename.rs

@@ -35,7 +35,7 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the final component of a path."
     }
 

crates/nu-command/src/path/dirname.rs

@@ -42,7 +42,7 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the parent directory of a path."
     }
 

crates/nu-command/src/path/exists.rs

@@ -33,11 +33,11 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Check whether a path exists."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"This only checks if it is possible to either `open` or `cd` to the given path.
 If you need to distinguish dirs and files, please use `path type`."#
     }

crates/nu-command/src/path/expand.rs

@@ -42,7 +42,7 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Try to expand a path to its absolute form."
     }
 

crates/nu-command/src/path/join.rs

@@ -33,11 +33,11 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Join a structured path or a list of path parts."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"Optionally, append an additional path to the result. It is designed to accept
 the output of 'path parse' and 'path split' subcommands."#
     }

crates/nu-command/src/path/parse.rs

@@ -32,11 +32,11 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert a path into structured data."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"Each path is split into a table with 'parent', 'stem' and 'extension' fields.
 On Windows, an extra 'prefix' column is added."#
     }

crates/nu-command/src/path/path_.rs

@@ -14,11 +14,11 @@ impl Command for PathCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Explore and manipulate paths."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"You must use one of the following subcommands. Using this command as-is will only produce this help message.
 
 There are three ways to represent a path:

crates/nu-command/src/path/relative_to.rs

@@ -35,11 +35,11 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Express a path as relative to another path."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"Can be used only when the input and the argument paths are either both
 absolute or both relative. The argument path needs to be a parent of the input
 path."#

crates/nu-command/src/path/split.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Split a path into a list based on the system's path separator."
     }
 

crates/nu-command/src/path/type.rs

@@ -31,11 +31,11 @@ impl Command for SubCommand {
             .category(Category::Path)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the type of the object a path refers to (e.g., file, dir, symlink)."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"This checks the file system to confirm the path's object type.
 If the path does not exist, null will be returned."#
     }