CC Mode. Allow fontification of "wrong" style comments with warning face.
This fixes bug #4192. * etc/NEWS: Add a new entry. * lisp/progmodes/cc-defs.el (c-font-lock-flush): New macro. * lisp/progmodes/cc-cmds.el (c-toggle-comment-style): On toggling the comment style, invoke c-font-lock-flush when c-mark-wrong-style-of-comment is non-nil, to cause that marking to be done instead on the other style of comment. * lisp/progmodes/cc-fonts.el (c-maybe-font-lock-wrong-style-comments): New function. (c-cpp-matchers): Call c-maybe-font-lock-wrong-style-comments when appropriate. * lisp/progmodes/cc-vars.el (c-mark-wrong-style-of-comment): New customizable option. * doc/misc/cc-mode.texi (top level, Indentation Commands, Guessing the Style, Custom Macros): For some opening quote marks, correct '' to ``. (Minor Modes): Add an xref to the new page "Wrong Comment Style" in a footnote. (Wrong Comment Style): New page.