docs: private Git submodules are not supported (#11876)

lib/manager/git-submodules/readme.md

-Keeps Git submodules updated within a repository.
+Keeps publicly accessible Git submodules updated within a repository.
+
+Renovate does not support updating Git submodules that are hosted on a private repository.
+Subscribe to [issue #10149 on GitHub](https://github.com/renovatebot/renovate/issues/10149) to keep track of our progress towards supporting private Git submodules.