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

src/main.rs

@@ -563,8 +563,13 @@ fn parse_commandline_args(
             let help = call.has_flag("help");
 
             if help {
-                let full_help =
-                    get_full_help(&Nu.signature(), &Nu.examples(), engine_state, &mut stack);
+                let full_help = get_full_help(
+                    &Nu.signature(),
+                    &Nu.examples(),
+                    engine_state,
+                    &mut stack,
+                    true,
+                );
 
                 let _ = std::panic::catch_unwind(move || stdout_write_all_and_flush(full_help));
 
@@ -600,7 +605,13 @@ fn parse_commandline_args(
     }
 
     // Just give the help and exit if the above fails
-    let full_help = get_full_help(&Nu.signature(), &Nu.examples(), engine_state, &mut stack);
+    let full_help = get_full_help(
+        &Nu.signature(),
+        &Nu.examples(),
+        engine_state,
+        &mut stack,
+        true,
+    );
     print!("{}", full_help);
     std::process::exit(1);
 }
@@ -731,7 +742,7 @@ impl Command for Nu {
         _input: PipelineData,
     ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
         Ok(Value::String {
-            val: get_full_help(&Nu.signature(), &Nu.examples(), engine_state, stack),
+            val: get_full_help(&Nu.signature(), &Nu.examples(), engine_state, stack, true),
             span: call.head,
         }
         .into_pipeline_data())

src/tests/test_engine.rs

@@ -53,7 +53,7 @@ fn in_and_if_else() -> TestResult {
 
 #[test]
 fn help_works_with_missing_requirements() -> TestResult {
-    run_test(r#"each --help | lines | length"#, "37")
+    run_test(r#"each --help | lines | length"#, "40")
 }
 
 #[test]