Consistent reference to RFCs in the documentation

When referencing RFC's, we have a mix of ulinking to the ietf.org entry and
not.  Also, for subsequent mentions of the same RFC on the same page we have
some as <acronym> while others are not.  The attached patch adds ulinks for all
RFC's and marks subsequent mentions as acronym to make the docs consistent.  It
also spells all as "RFC <number>" with a whitespace as that was the most
commonly used spelling (there is no RFC for how to reference to an RFC so we're
free to choose).

In order to make review easier I haven't fixed linelengths/wrapping, but am
happy to do that in case this is deemed something we want.

cheers ./daniel