Per issue 7900, os.getgroups on OSX does not behave the same way as on any other unix platform.  This seems worthy of a documentation note, since anyone trying to write portable code could get bit by this.

I don't really understand the relationship on OSX between what the current os.getgroups returns, what the normal unix os.getgroups returns, and what things a process can actually *do*, so I can't write that documentation.  Hopefully someone else can.

A doc note is probably also needed about the relationship between os.setgroups and os.getgroups on OSX, which again I do not understand and so cannot write.