docs: Improve help_heading documentation

- clarify the usage of `help_heading` and `subcommand_help_heading`

Author: gortavoher

clap_builder/src/builder/command.rs

@@ -3703,10 +3703,10 @@ impl Command {
         self
     }
 
-    /// Set a custom [`help_heading`] for the command
+    /// Set a custom help heading for the command
     ///
     /// To place the `help` subcommand under a custom heading amend the default
-    /// heading using `subcommand_help_heading`.
+    /// heading using [`Command::subcommand_help_heading`].
     ///
     /// # Examples
     ///