submodule doc: reorder introductory paragraphs

It's better to start the man page with a description of what
submodules actually are, instead of saying what they are not.

Reorder the paragraphs such that

 - the first short paragraph introduces the submodule concept,
 - the second paragraph highlights the usage of the submodule command,
 - the third paragraph giving background information, and finally
 - the fourth paragraph discusing alternatives such as subtrees and
   remotes, which we don't want to be confused with.

This ordering deepens the knowledge on submodules with each paragraph.
First the basic questions like "How/what" will be answered, while the
underlying concepts will be taught at a later time.

Making sure it is not confused with subtrees and remotes is not really
enhancing knowledge of submodules itself, but rather painting the big
picture of git concepts, so you could also argue to have it as the second
paragraph. Personally I think this may confuse readers, specially
newcomers though.

Additionally to reordering the paragraphs, they have been slightly
reworded.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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