Document some Emacs 24.3 changes.

Document some Emacs 24.3 changes.

* doc/emacs/dired.texi (Dired Deletion, Marks vs Flags): Document Emacs 24.3
changes to the mark and unmark commands.
(Comparison in Dired): Document chages to dired-diff.  Remove M-=,
which is no longer bound to dired-backup-diff.

* doc/emacs/mule.texi (Text Coding): set-buffer-file-coding-system can now
be invoked from the mode line.

* lisp/dired.el (dired-mark, dired-unmark, dired-flag-file-deletion):
Doc fix.