Update docs wrt shr/html mail.

* gnus.texi (HTML): Update section so that it mentions shr and w3m.

* gnus-faq.texi (FAQ 4 - Reading messages, FAQ 4-16): Add Q&A on how to
increase contrast when displaying HTML mail with shr.
Add link to EWW manual.
gnus.texi (HTML): Fix xref to EWW
Fix docstring typo.

* gnus-start.el (gnus-activate-group): Fix typo reported by Tim
Landscheidt.
gnus.texi (Top): Add missing `HTML' menu.
(HTML): Fix xref to FAQ 4-16.
mml2015.el (mml2015-display-key-image): Adjust :version
Allow uploading files from eww

2014-11-10 Kenjiro NAKAYAMA <nakayamakenjiro@gmail.com>
  * mm-url.el (mm-url-encode-multipart-form-data):
  Restore to handle "multipart/form-data" by eww.
gnus-notifications: use both photo and app-icon
ietf-drums.el (ietf-drums-parse-address): Don't issue warnings about
Don't query about discarding changes in ephemeral groups