Add link to correct documentation in htmldocck.py

Author: Derukugis

src/etc/htmldocck.py

@@ -2,6 +2,9 @@
 # -*- coding: utf-8 -*-
 
 r"""
+For docmentation and usage instructions, please see
+https://rustc-dev-guide.rust-lang.org/rustdoc-internals.html#dotting-is-and-crossing-ts
+
 htmldocck.py is a custom checker script for Rustdoc HTML outputs.
 
 # How and why?