Clarify Fragment#setUserVisibleHint guarantees in docs

Note that setUserVisibleHint can be called from outside of the
fragment lifecycle. Some apps override it and expect otherwise,
and the bug fix for N will affect their expectations.

https://code.google.com/p/android/issues/detail?id=213681

Bug 29526496

Change-Id: I68d5690cefe5f354313f3d89e8ec5ebccaf2544f