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/date/date_.rs

@@ -14,11 +14,11 @@ impl Command for Date {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Date-related commands."
     }
 
-    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."
     }
 

crates/nu-command/src/date/humanize.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Print a 'humanized' format for the date, relative to now."
     }
 

crates/nu-command/src/date/list_timezone.rs

@@ -15,7 +15,7 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "List supported time zones."
     }
 

crates/nu-command/src/date/now.rs

@@ -15,7 +15,7 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the current date."
     }
 

crates/nu-command/src/date/to_record.rs

@@ -20,7 +20,7 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert the date into a record."
     }
 

crates/nu-command/src/date/to_table.rs

@@ -20,7 +20,7 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert the date into a structured table."
     }
 

crates/nu-command/src/date/to_timezone.rs

@@ -19,11 +19,11 @@ impl Command for SubCommand {
             .category(Category::Date)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert a date to a given time zone."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Use 'date list-timezone' to list all supported time zones."
     }