Bind M-G unconditionally in Dired

* lisp/dired-x.el: Move dired-goto-subdir binding from here...
* lisp/dired.el (dired-mode-map): ...to here.  (Bug#21981)

* doc/misc/dired-x.texi (Miscellaneous Commands): Move
documentation of above command from here...
* doc/emacs/dired.texi (Subdirectory Motion): ...to here.

doc/emacs/dired.texi

@@ -1333,6 +1333,12 @@ parent directory.
 @kindex > @r{(Dired)}
 @item >
 Move down to the next directory-file line (@code{dired-next-dirline}).
+
+@findex dired-goto-subdir
+@kindex M-G @r{(Dired)}
+@item M-G
+Prompt for a directory and move to its directory-file line
+(@code{dired-goto-subdir}).
 @end table
 
 @node Hiding Subdirectories

doc/misc/dired-x.texi

@@ -882,15 +882,6 @@ normal and a wildcard buffer for the same directory, @kbd{C-x d @key{RET}}
 will toggle between those two.
 @end table
 
-@table @kbd
-@findex dired-goto-subdir
-@kindex M-G
-@item M-G
-(@code{dired-goto-subdir}) Go to the header line of an inserted directory.
-This command reads its argument, with completion derived from the names of the
-inserted subdirectories.
-@end table
-
 @table @code
 
 @item dired-vm

etc/NEWS

@@ -1065,6 +1065,11 @@ The corresponding key "Y" is now bound by default in Dired.
 *** 'dired-do-relsymlink-regexp' moved from dired-x to dired.
 The corresponding key "% Y" is now bound by default in Dired.
 
+---
+*** 'M-G' is now bound to 'dired-goto-subdir'.
+Before, that binding was only available if the 'dired-x' package was
+loaded.
+
 +++
 *** 'dired-info' and 'dired-man' moved from dired-x to dired.
 The 'dired-info' and 'dired-man' commands have been moved from the

lisp/dired-x.el

@@ -236,7 +236,6 @@ to nil: a pipe using `zcat' or `gunzip -c' will be used."
 (define-key dired-mode-map "\C-x\M-o" 'dired-omit-mode)
 (define-key dired-mode-map "\M-(" 'dired-mark-sexp)
 (define-key dired-mode-map "\M-!" 'dired-smart-shell-command)
-(define-key dired-mode-map "\M-G" 'dired-goto-subdir)
 (define-key dired-mode-map "F" 'dired-do-find-marked-files)
 (define-key dired-mode-map "V" 'dired-do-run-mail)
 

lisp/dired.el

@@ -2174,6 +2174,7 @@ Do so according to the former subdir alist OLD-SUBDIR-ALIST."
   "S-SPC"   #'dired-previous-line
   "<remap> <next-line>"        #'dired-next-line
   "<remap> <previous-line>"    #'dired-previous-line
+  "M-G"    #'dired-goto-subdir
   ;; hiding
   "$"       #'dired-hide-subdir
   "M-$"     #'dired-hide-all