[PATCH 0/2] Improve "git submodule add" documentation.

This series applies atop Jens's 2/3 patch.  Jens, please feel free to
squash these into your commit if you like.

The first commit makes the documentation (hopefully) more accurately
describe how git chooses which superproject remote to use.

The second commit moves the paragraph describing the utility of relative
submodule URLs right after their description, making it more likely for
readers to see it (instead of assuming it's part of the <path> parameter's
documentation -- as I did on previous occasions).

		M.