Workaround and document #action #ahah bug

Take a look at #591696: Problem with various AHAH forms and validation and http://jbenner.net/blog/prevent-ahah-the-right-way-from-breaking-with-va...

I'm not sure if we can fix this completely within this code, or simply stash #action and leave it up to the calling module to fix it up. Either way, we should document it until it's fixed in core.