document 'allow-tip-sha1-in-want' capability

See 390eb36 (upload-pack: optionally allow fetching from the tips of
hidden refs - 2013-01-28) for more information.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>