MarcFieldDocURL is really great feature. In this time we hasn't linked help for authorities, just for bibliographics records. So, if we add some kind of paramether based on type of records (like b=biblio, or a=authorities) we can link LOC or other online help for fields.
Bug 21280 does add exactly this in its patchset
*** This bug has been marked as a duplicate of bug 21280 ***