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/generators/cal.rs

@@ -54,7 +54,7 @@ impl Command for Cal {
             .category(Category::Generators)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Display a calendar."
     }
 

crates/nu-command/src/generators/generate.rs

@@ -22,11 +22,11 @@ impl Command for Generate {
             .category(Category::Generators)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Generate a list of values by successively invoking a closure."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"The generator closure accepts a single argument and returns a record
 containing two optional keys: 'out' and 'next'. Each invocation, the 'out'
 value, if present, is added to the stream. If a 'next' key is present, it is

crates/nu-command/src/generators/seq.rs

@@ -16,7 +16,7 @@ impl Command for Seq {
             .category(Category::Generators)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Output sequences of numbers."
     }
 

crates/nu-command/src/generators/seq_char.rs

@@ -8,7 +8,7 @@ impl Command for SeqChar {
         "seq char"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Print a sequence of ASCII characters."
     }
 

crates/nu-command/src/generators/seq_date.rs

@@ -11,7 +11,7 @@ impl Command for SeqDate {
         "seq date"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Print sequences of dates."
     }