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

tests/repl/test_parser.rs

@@ -249,31 +249,31 @@ fn recursive_parse() -> TestResult {
 }
 
 #[test]
-fn commands_have_usage() -> TestResult {
+fn commands_have_description() -> TestResult {
     run_test_contains(
         r#"
     # This is a test
     #
-    # To see if I have cool usage
+    # To see if I have cool description
     def foo [] {}
     help foo"#,
-        "cool usage",
+        "cool description",
     )
 }
 
 #[test]
-fn commands_from_crlf_source_have_short_usage() -> TestResult {
+fn commands_from_crlf_source_have_short_description() -> TestResult {
     run_test_contains(
-        "# This is a test\r\n#\r\n# To see if I have cool usage\r\ndef foo [] {}\r\nscope commands | where name == foo | get usage.0",
+        "# This is a test\r\n#\r\n# To see if I have cool description\r\ndef foo [] {}\r\nscope commands | where name == foo | get description.0",
         "This is a test",
     )
 }
 
 #[test]
-fn commands_from_crlf_source_have_extra_usage() -> TestResult {
+fn commands_from_crlf_source_have_extra_description() -> TestResult {
     run_test_contains(
-        "# This is a test\r\n#\r\n# To see if I have cool usage\r\ndef foo [] {}\r\nscope commands | where name == foo | get extra_usage.0",
-        "To see if I have cool usage",
+        "# This is a test\r\n#\r\n# To see if I have cool description\r\ndef foo [] {}\r\nscope commands | where name == foo | get extra_description.0",
+        "To see if I have cool description",
     )
 }