Thanks, I have a couple of minor comments below.
* doc/misc/dired-x.texi: Update documentation for 'dired-mark-extension'.
This part of the log entry should state in parentheses the node in
which the change is being made, as if the node were a function.
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.
+Calling it with the @kbd{C-u C-u} prefix, asks for a character to use
^^^^^^^^^^
"If called" sounds better and more correct English-wise.
+as the marker, and mark files with it.
^^^^
"marks"
+Prefixed with two C-u’s, prompt for MARKER-CHAR and mark files with it."
^
Please use the ASCII apostrophe character ' here, not ’, so that the
user could control the result via text-quoting-style.
With the above gotchas fixed, this is okay for the master branch.