Documentation/git-clone: mention progress in -v

Documentation/git-clone: mention progress in -v

After 5a518ad (clone: use --progress to force progress reporting),
-v/--verbose did not affect whether progress status was reported to
stderr, and users accustomed to using -v to do so since 21188b1
(Implement git clone -v) may be confused.

Mitigate such risks by stating -v does not affect progress in the
documentation.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>