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/date/date_.rs

@@ -53,7 +53,13 @@ fn date(
     let head = call.head;
 
     Ok(Value::String {
-        val: get_full_help(&Date.signature(), &Date.examples(), engine_state, stack),
+        val: get_full_help(
+            &Date.signature(),
+            &Date.examples(),
+            engine_state,
+            stack,
+            false,
+        ),
         span: head,
     }
     .into_pipeline_data())