docs: Updates to multi-window and related docs.

Clarified behavior when activity is resized or put in fullscreen
mode, per b/28580007 . Also (per email from o-o) removed misleading
statement about when onStop() might or might not be called.

Both changes can go live now (multiwindow update applies to DP1 & 2,
and onStop() clarification applies to all versions of API), so I'll
submit as soon as this is approved.

See first comment for doc stage location.

bug: 28580007
Change-Id: Ib008f24e5796ec7810b67c91e512e679680d4afd