backout: document return code of merge conflict

mercurial/commands.py

@@ -416,7 +416,7 b' def backout(ui, repo, node=None, rev=Non'
 
     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"))