Merge branch 'pb/submodule-doc-xref'

Doc update.

* pb/submodule-doc-xref:
  gitmodules: link to gitsubmodules guide

Author: gitster

Documentation/gitmodules.txt

@@ -120,7 +120,7 @@ submodules a URL is specified which can be used for cloning the submodules.
 
 SEE ALSO
 --------
-linkgit:git-submodule[1] linkgit:git-config[1]
+linkgit:git-submodule[1], linkgit:gitsubmodules[7], linkgit:git-config[1]
 
 GIT
 ---