document submdule.$name.update=none option for gitmodules

This option was not yet described in the gitmodules documentation. We
only described it in the 'git submodule' command documentation but
gitmodules is the more natural place to look.

A short reference in the 'git submodule' documentation should be
sufficient since the details can now be found in the documentation to
gitmodules.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -140,7 +140,8 @@ update::
 	checkout the commit specified in the index of the containing repository.
 	This will make the submodules HEAD be detached unless `--rebase` or
 	`--merge` is specified or the key `submodule.$name.update` is set to
-	`rebase`, `merge` or `none`.
+	`rebase`, `merge` or `none`. `none` can be overriden by specifying
+	`--checkout`.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the
@@ -148,10 +149,6 @@ submodule with the `--init` option.
 +
 If `--recursive` is specified, this command will recurse into the
 registered submodules, and update any nested submodules within.
-+
-If the configuration key `submodule.$name.update` is set to `none` the
-submodule with name `$name` will not be updated by default. This can be
-overriden by adding `--checkout` to the command.
 
 summary::
 	Show commit summary between the given commit (defaults to HEAD) and

Documentation/gitmodules.txt

@@ -41,8 +41,11 @@ submodule.<name>.update::
 	the commit specified in the superproject. If 'merge', the commit
 	specified in the superproject will be merged into the current branch
 	in the submodule.
+	If 'none', the submodule with name `$name` will not be updated
+	by default.
+
 	This config option is overridden if 'git submodule update' is given
-	the '--merge' or '--rebase' options.
+	the '--merge', '--rebase' or '--checkout' options.
 
 submodule.<name>.fetchRecurseSubmodules::
 	This option can be used to control recursive fetching of this