doc/misc/{gnus.texi,gnus-faq.texi}: Update docs wrt shr/html mail

* gnus.texi (HTML): Update section so that it mentions shr and w3m.
Also link the full EWW manual that explains more on shr, too.
* gnus-faq.texi (FAQ 4 - Reading messages, FAQ 4-16): Add Q&A on how to
increase contrast when displaying HTML mail with shr.
doc/misc/ChangeLog: Fix date for r117699