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-explore/src/commands/expand.rs

@@ -30,7 +30,7 @@ impl ViewCommand for ExpandCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/commands/help.rs

@@ -91,7 +91,7 @@ impl ViewCommand for HelpCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/commands/mod.rs

@@ -24,7 +24,7 @@ pub use table::TableCmd;
 pub trait SimpleCommand {
     fn name(&self) -> &'static str;
 
-    fn usage(&self) -> &'static str;
+    fn description(&self) -> &'static str;
 
     fn parse(&mut self, args: &str) -> Result<()>;
 
@@ -42,7 +42,7 @@ pub trait ViewCommand {
 
     fn name(&self) -> &'static str;
 
-    fn usage(&self) -> &'static str;
+    fn description(&self) -> &'static str;
 
     fn parse(&mut self, args: &str) -> Result<()>;
 

crates/nu-explore/src/commands/nu.rs

@@ -33,7 +33,7 @@ impl ViewCommand for NuCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/commands/quit.rs

@@ -18,7 +18,7 @@ impl SimpleCommand for QuitCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/commands/table.rs

@@ -36,7 +36,7 @@ impl ViewCommand for TableCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/commands/try.rs

@@ -28,7 +28,7 @@ impl ViewCommand for TryCmd {
         Self::NAME
     }
 
-    fn usage(&self) -> &'static str {
+    fn description(&self) -> &'static str {
         ""
     }
 

crates/nu-explore/src/explore.rs

@@ -17,7 +17,7 @@ impl Command for Explore {
         "explore"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Explore acts as a table pager, just like `less` does for text."
     }
 
@@ -47,7 +47,7 @@ impl Command for Explore {
             .category(Category::Viewers)
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"Press `:` then `h` to get a help menu."#
     }
 

crates/nu-explore/src/registry/command.rs

@@ -65,8 +65,8 @@ where
         self.0.name()
     }
 
-    fn usage(&self) -> &'static str {
-        self.0.usage()
+    fn description(&self) -> &'static str {
+        self.0.description()
     }
 
     fn parse(&mut self, args: &str) -> Result<()> {