add note about "markupbase" not being intended for direct use

(closes SF bug #736659, patch #901369; backport of Lib/markupbase.py 1.11)

Lib/markupbase.py

-"""Shared support for scanning document type declarations in HTML and XHTML."""
+"""Shared support for scanning document type declarations in HTML and XHTML.
+
+This module is used as a foundation for the HTMLParser and sgmllib
+modules (indirectly, for htmllib as well).  It has no documented
+public API and should not be used directly.
+
+"""
 
 import re