mention -n

Doc/library/2to3.rst

@@ -37,8 +37,8 @@ It can be converted to Python 3.x code via 2to3 on the command line::
 
 A diff against the original source file is printed.  2to3 can also write the
 needed modifications right back to the source file.  (Of course, a backup of the
-original is also be made.)  Writing the changes back is enabled with the
-:option:`-w` flag::
+original is also be made unless :option:`-n` is also given.)  Writing the
+changes back is enabled with the :option:`-w` flag::
 
    $ 2to3 -w example.py