Add documentation for show-branch --topics
Add documentation for show-branch --topics

Add a quick paragraph explaining the --topics option for show-branch.
The explanation is an abbreviated version of the commit message from
d320a5437f8304cf9ea3ee1898e49d643e005738.

Signed-off-by: Brian Gernhardt <9e7bc6878f15155f664887f5952d257c0d032745@silverinsanity.com>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git