docs: clarify that passing --depth to git-clone implies --single-branch

It is confusing to document how --depth behaves as part of the
--single-branch docs. Better move that part to the --depth docs, saying
that it implies --single-branch by default.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>