add signature information when get help on one command (#7079)

* add signature information when help on one command

* tell user that one command support operated on cell paths

Also, make type output to be more friendly, like `record<>` should just be `record`

And the same to `table<>`, which should be `table`

* simplify code

* don't show signatures for parser keyword

* update comment

* output arg syntax shape as type, so it's the same as describe command

* fix string when no positional args

* update signature body

* update

* add help signature test

* fix arg output format for composed data type like list or record

* fix clippy

* add comment

crates/nu-command/src/platform/reedline_commands/keybindings.rs

@@ -38,6 +38,7 @@ impl Command for Keybindings {
                 &Keybindings.examples(),
                 engine_state,
                 stack,
+                self.is_parser_keyword(),
             ),
             span: call.head,
         }