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>

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