Documentation/git-submodule.txt: Further clarify the description

Documentation/git-submodule.txt: Further clarify the description

This patch rewrites the general description yet again, first clarifying
the high-level concept, mentioning the difference to remotes and using
the subtree merge strategy, then getting to the details about tree
entries and .gitmodules file.

The patch also makes few smallar grammar fixups within the rest of the
description and clarifies how does 'init' relate to 'update --init'.

Cc: Heikki Orsila <shdl@zakalwe.fi>
Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>