corrected and clarified some stuff related to action functions

[Gutawer]

I was looking through the GZDoom source to work out some stuff about the internals of the action function system and after checking these docs I noticed that they don't get certain things right (or aren't specific enough). So I've tried to create a more technically complete definition here based on the results of an hour or so of playing around with ZScript and reading the source code. I may not have gotten everything completely perfect but I think it's a more complete description of the messy parts of the system.