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/tests/commands/reject.rs

@@ -92,7 +92,7 @@ fn reject_record_from_raw_eval() {
         )
     );
 
-    assert!(actual.out.contains("record<>"));
+    assert!(actual.out.contains("record"));
 }
 
 #[test]