Add help text to permissions options

[agilbert]

The group permissions system is extremely powerful and configurable at a very detailed level.

But when configuring permissions, there is no information present on how to use them in UI, thus making it unlikely a site admin will be able to understand and take advantage of these features.

[boutell]

Closing because the new extended permissions feature will bury this one pretty solidly and the legacy permissions, while available, will not get new love.