Work around an rfc2629.xslt idiosyncrasy
1.0.21: Hijack the previously broken abbrev feature for auto-indexing of certain words.
Example usage for indexing two BCP14 words as subterms under a common term "BCP14":

*[MUST]: BCP14
*[MUST NOT]: BCP14

(This breaks when the search term occurs in a heading.  More general access to xml2rfc indexing is a "to do", anyway.)