Describe '-p' as a short form of '--patch' in synopsis and options. Also
refer the reader to the patch mode description of git-add documentation.
Helped-by: Jeff King <peff@peff.net> Mentored-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Valentin Haenel <valentin.haenel@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>