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
d320a5437f.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-show-branch.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 [verse]
 'git-show-branch' [--all] [--heads] [--tags] [--topo-order] [--current]
 		[--more=<n> | --list | --independent | --merge-base]
-		[--no-name | --sha1-name] [<rev> | <glob>]...
+		[--no-name | --sha1-name] [--topics] [<rev> | <glob>]...
 
 DESCRIPTION
 -----------
@@ -86,6 +86,14 @@ OPTIONS
 	of "master"), name them with the unique prefix of their
 	object names.
 
+--topics::
+	Shows only commits that are NOT on the first branch given.
+	This helps track topic branches by hiding any commit that
+	is already in the main line of development.  When given
+	"git show-branch --topics master topic1 topic2", this
+	will show the revisions given by "git rev-list {caret}master
+	topic1 topic2"
+
 Note that --more, --list, --independent and --merge-base options
 are mutually exclusive.