Better describe how repository permissions work in conjunction with project permissions

From STASH-3508

I. The documentation on Project vs. Repo permissions is poor. I understand that you guys want to keep this simple, but this doesn't give me enough information to predict what combinations of Project (P) and Repo (R) permissions will do:

https://confluence.atlassian.com/display/STASH/Using+repository+permissions

In particular there's nothing about whether repo permissions override project permissions or vice versa. This should be made VERY clear in the docs. The only place I see it listed is in the one sentence at the top of the repo permissions screen, but some example in the docs would be really useful.