user-manual: Flesh out uncommitted changes and submodule updates

If you try and update a submodule with a dirty working directory, you
get an error message like:

  $ git submodule update
  error: Your local changes to the following files would be overwritten by checkout:
  ...
  Please, commit your changes or stash them before you can switch branches.
  Aborting
  ...

Mention this in the submodule notes.  The previous phrase was short
enough that I originally thought it might have been referring to the
reflog note (obviously, uncommitted changes will not show up in the
reflog either ;).

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -3739,7 +3739,9 @@ module a
 
 NOTE: The changes are still visible in the submodule's reflog.
 
-This is not the case if you did not commit your changes.
+If you have uncommitted changes in your submodule working tree, `git
+submodule update` will not overwrite them.  Instead, you get the usual
+warning about not being able switch from a dirty branch.
 
 [[low-level-operations]]
 Low-level git operations