Prefer "this" to “this” in doc strings

Prefer "this" to “this” in doc strings

This mostly just straightens quotes introduced in my previous patch.
Suggested by Dmitry Gutov in:
http://lists.gnu.org/archive/html/emacs-devel/2015-05/msg00565.html
* lisp/faces.el, lisp/gnus/gnus-group.el, lisp/ldefs-boot.el:
* lisp/mail/supercite.el, lisp/net/tramp.el, lisp/recentf.el:
* lisp/textmodes/artist.el, lisp/textmodes/rst.el:
* lisp/textmodes/tildify.el, lisp/vc/ediff-util.el:
* lisp/vc/log-edit.el, lisp/xt-mouse.el:
Prefer straight double quotes to curved double quotes in doc strings.