Document permission operator precedence rules

[tstirrat15]

If not already documented. We've had a couple of reports of folks losing time because they make an assumption that permission operators behave like arithmetic operators rather than set-theoretic operators, which don't have the same conventions of precedence.

[patrick-th]

Set theory was quite a while ago for me, but in case the current implementation is mathematically correct, may I suggest adding a runtime warning in cases where one could expect the "usual" and-before-or precedence and there aren't any parentheses?