Documentation: add submodule.* to the big configuration variable list

The url, path, and the update items in [submodule "foo"] stanzas
are nicely explained in the .gitmodules and ‘git submodule’
documentation.  Point there from the config documentation.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1685,6 +1685,15 @@ status.submodulesummary::
 	summary of commits for modified submodules will be shown (see
 	--summary-limit option of linkgit:git-submodule[1]).
 
+submodule.<name>.path::
+submodule.<name>.url::
+submodule.<name>.update::
+	The path within this project, URL, and the updating strategy
+	for a submodule.  These variables are initially populated
+	by 'git submodule init'; edit them to override the
+	URL and other values found in the `.gitmodules` file.  See
+	linkgit:git-submodule[1] and linkgit:gitmodules[5] for details.
+
 tar.umask::
 	This variable can be used to restrict the permission bits of
 	tar archive entries.  The default is 0002, which turns off the