user-manual: Explain what submodules are good for.
user-manual: Explain what submodules are good for.

Rework the introduction to the Submodules section to explain why
someone would use them, and fix up submodule references from the
tree-object and todo sections.

Signed-off-by: Michael Smith <3f5005aab17d0ac4ac1327237c53ad461fb34e8a@cbnco.com>
Acked-by: J. Bruce Fields <51738506c1b2ccb0761f23bdc612c93babf738ea@citi.umich.edu>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git