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/help.rs

@@ -14,3 +14,15 @@ fn help_commands_length() {
     let is_positive = output_int.is_positive();
     assert!(is_positive);
 }
+
+#[test]
+fn help_shows_signature() {
+    let actual = nu!(cwd: ".", pipeline("help str distance"));
+    assert!(actual
+        .out
+        .contains("<string> | str distance <string> -> <int>"));
+
+    // don't show signature for parser keyword
+    let actual = nu!(cwd: ".", pipeline("help alias"));
+    assert!(!actual.out.contains("Signatures"));
+}