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 v24 will affect their expectations.

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

Bug 29526496

Change-Id: I401d1b5353aa1c3693ff728283ff81cf61b14212
1 file changed