docs: Noted that custom permissions must be unique per-device

We'd already noted this in the "new in Android 5.0" page but we
hadn't moved this into the base "permissions" pages. Added it, with
some usage recommendations from the bug.

Also fixed some formatting while I had the files open.

See first comment for doc stage location.

bug: 18143176
Change-Id: I9cc1a8865492ebf30c7a0f7c325669ded670933e