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/strings/split/command.rs

@@ -34,6 +34,7 @@ impl Command for SplitCommand {
                 &SplitCommand.examples(),
                 engine_state,
                 stack,
+                self.is_parser_keyword(),
             ),
             span: call.head,
         }

crates/nu-command/src/strings/str_/case/str_.rs

@@ -29,7 +29,13 @@ impl Command for Str {
         _input: PipelineData,
     ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
         Ok(Value::String {
-            val: get_full_help(&Str.signature(), &Str.examples(), engine_state, stack),
+            val: get_full_help(
+                &Str.signature(),
+                &Str.examples(),
+                engine_state,
+                stack,
+                self.is_parser_keyword(),
+            ),
             span: call.head,
         }
         .into_pipeline_data())