backout: document return code of merge conflict stable

backout: document return code of merge conflict

--- a/mercurial/commands.py	Tue Apr 01 02:46:03 2014 +0900
+++ b/mercurial/commands.py	Sat Mar 08 18:41:56 2014 +0900
@@ -416,7 +416,7 @@
 
     See :hg:`help dates` for a list of formats valid for -d/--date.
 
-    Returns 0 on success.
+    Returns 0 on success, 1 if there are unresolved files.
     '''
     if rev and node:
         raise util.Abort(_("please specify just one revision"))