Clarify Gravatar docs
For discussion, see the following thread: https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00528.html * doc/misc/gnus.texi (X-Face): Fix cross-reference. (Gravatars): * lisp/gnus/gnus-gravatar.el (gnus-gravatar-too-ugly): * lisp/image/gravatar.el (gravatar-cache-ttl, gravatar-rating) (gravatar-size): Clarify user option descriptions. (gravatar-retrieve, gravatar-retrieve-synchronously): Document return value.