git-cherry: document limit and add diagram

git-cherry: document limit and add diagram

This patch adds the diagram from the long usage string of git-cherry to
its documentation, and documents the third option.  I changed some of
the + to - in order to save the reader from wondering where they might
fit into the picture.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>