whatchanged: document -m option from git-diff-tree.

The documentation for git-whatchanged is meant to describe only
the most frequently used options from git-diff-tree.  Because "why
doesn't it show merges" was asked more than once, we'd better
describe '-m' option there.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-whatchanged.txt

@@ -44,6 +44,13 @@ OPTIONS
 	<format> can be one of 'raw', 'medium', 'short', 'full',
 	and 'oneline'.
 
+-m::
+	By default, differences for merge commits are not shown.
+	With this flag, show differences to that commit from all
+	of its parents.
+
+	However, it is not very useful in general, although it
+	*is* useful on a file-by-file basis.
 
 Author
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