Issue 9185 - Add note about when -op is useful.

http://d.puremagic.com/issues/show_bug.cgi?id=9185

This should help out avoid reports like [8808](http://d.puremagic.com/issues/show_bug.cgi?id=8808) and [4495](http://d.puremagic.com/issues/show_bug.cgi?id=4495).