mdiff.diffopts: add doc comment for nobinary

mdiff.diffopts: add doc comment for nobinary

--- a/mercurial/mdiff.py	Sat Nov 08 17:08:09 2014 +0000
+++ b/mercurial/mdiff.py	Wed Nov 12 23:19:44 2014 -0800
@@ -25,6 +25,7 @@
     showfunc enables diff -p output
     git enables the git extended patch format
     nodates removes dates from diff headers
+    nobinary ignores binary files
     ignorews ignores all whitespace changes in the diff
     ignorewsamount ignores changes in the amount of whitespace
     ignoreblanklines ignores changes whose lines are all blank