docs: Added missing REQUEST_IGNORE_BATTERY_OPTIMIZATIONS permission

While checking the manifest for N permissions changes, I saw a
normal permission that had been added in M and not documented.
(The permission was added to AndroidManifest.xml, so it *was*
being listed in the android.Manifest javadoc.)

See first comment for doc stage location.

bug: 27222922
Change-Id: I5740fdb3da3cac37b15d87fe5f092d305c52449d