Added a note on the meaning of (prev_time, current_time) arguments for ``op`` function.
Added a note on the meaning of (prev_time, current_time) arguments for ``op`` function.

--HG--
extra : convert_revision : tzeentch.gm%40gmail.com-20111004213417-hd0jbx7uirou6vcw

swayf/doit,swayf/doit,okin/doit,swayf/doit,okin/doit,okin/doit