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-engine/src/documentation.rs

@@ -76,15 +76,15 @@ fn get_documentation(
     let cmd_name = &sig.name;
     let mut long_desc = String::new();
 
-    let usage = &sig.usage;
-    if !usage.is_empty() {
-        long_desc.push_str(usage);
+    let desc = &sig.description;
+    if !desc.is_empty() {
+        long_desc.push_str(desc);
         long_desc.push_str("\n\n");
     }
 
-    let extra_usage = &sig.extra_usage;
-    if !extra_usage.is_empty() {
-        long_desc.push_str(extra_usage);
+    let extra_desc = &sig.extra_description;
+    if !extra_desc.is_empty() {
+        long_desc.push_str(extra_desc);
         long_desc.push_str("\n\n");
     }
 
@@ -118,7 +118,7 @@ fn get_documentation(
         {
             subcommands.push(format!(
                 "  {help_subcolor_one}{}{RESET} - {}",
-                sig.name, sig.usage
+                sig.name, sig.description
             ));
         }
     }

crates/nu-engine/src/scope.rs

@@ -109,12 +109,12 @@ impl<'e, 's> ScopeData<'e, 's> {
                     "name" => Value::string(String::from_utf8_lossy(command_name), span),
                     "category" => Value::string(signature.category.to_string(), span),
                     "signatures" => self.collect_signatures(&signature, span),
-                    "usage" => Value::string(decl.usage(), span),
+                    "description" => Value::string(decl.description(), span),
                     "examples" => Value::list(examples, span),
                     "type" => Value::string(decl.command_type().to_string(), span),
                     "is_sub" => Value::bool(decl.is_sub(), span),
                     "creates_scope" => Value::bool(signature.creates_scope, span),
-                    "extra_usage" => Value::string(decl.extra_usage(), span),
+                    "extra_description" => Value::string(decl.extra_description(), span),
                     "search_terms" => Value::string(decl.search_terms().join(", "), span),
                     "decl_id" => Value::int(**decl_id as i64, span),
                 };
@@ -332,7 +332,7 @@ impl<'e, 's> ScopeData<'e, 's> {
             if decl.is_known_external() {
                 let record = record! {
                     "name" => Value::string(String::from_utf8_lossy(command_name), span),
-                    "usage" => Value::string(decl.usage(), span),
+                    "description" => Value::string(decl.description(), span),
                     "decl_id" => Value::int(**decl_id as i64, span),
                 };
 
@@ -366,7 +366,7 @@ impl<'e, 's> ScopeData<'e, 's> {
                         record! {
                             "name" => Value::string(String::from_utf8_lossy(&decl_name), span),
                             "expansion" => Value::string(expansion, span),
-                            "usage" => Value::string(alias.usage(), span),
+                            "description" => Value::string(alias.description(), span),
                             "decl_id" => Value::int(decl_id as i64, span),
                             "aliased_decl_id" => aliased_decl_id,
                         },
@@ -470,9 +470,9 @@ impl<'e, 's> ScopeData<'e, 's> {
         sort_rows(&mut export_submodules);
         sort_rows(&mut export_consts);
 
-        let (module_usage, module_extra_usage) = self
+        let (module_desc, module_extra_desc) = self
             .engine_state
-            .build_module_usage(*module_id)
+            .build_module_desc(*module_id)
             .unwrap_or_default();
 
         Value::record(
@@ -484,8 +484,8 @@ impl<'e, 's> ScopeData<'e, 's> {
                 "submodules" => Value::list(export_submodules, span),
                 "constants" => Value::list(export_consts, span),
                 "has_env_block" => Value::bool(module.env_block.is_some(), span),
-                "usage" => Value::string(module_usage, span),
-                "extra_usage" => Value::string(module_extra_usage, span),
+                "description" => Value::string(module_desc, span),
+                "extra_description" => Value::string(module_extra_desc, span),
                 "module_id" => Value::int(*module_id as i64, span),
             },
             span,