dired-mark-extension: Unmark if called with C-u prefix

See discussion in #Bug2518 and:
https://lists.gnu.org/archive/html/emacs-devel/2016-09/msg00711.html
* lisp/dired-x.el (dired-mark-extension):
Update interactive calls: a prefix arg C-u unmark files;
a prefix C-u C-u prompt for MARKER-CHAR and mark files with it.
(dired-mark-sexp):
Show in the prompt that we are unmarking if called with a prefix argument.
* doc/misc/dired-x.texi (Advanced Mark Commands):
Update documentation for 'dired-mark-extension'.

1 files changed, 4 insertions, 2 deletions

diff --git a/doc/misc/dired-x.texi b/doc/misc/dired-x.texi
index 2391852ca0f..db01896095c 100644
--- a/doc/misc/dired-x.texi
+++ b/doc/misc/dired-x.texi
@@ -710,8 +710,10 @@ variable @code{window-min-height}.
 @findex dired-mark-extension
 Mark all files with a certain extension for use in later commands.  A @samp{.}
 is not automatically prepended to the string entered, you must type it
-explicitly.  If invoked with a prefix argument, this command asks for
-a character to use as the marker.
+explicitly.
+If invoked with prefix argument @kbd{C-u}, this command unmark files instead.
+If called with the @kbd{C-u C-u} prefix, asks for a character to use
+as the marker, and marks files with it.
 
 When called from Lisp, @var{extension} may also be a list of extensions
 and an optional argument @var{marker-char} specifies the marker used.