docs: Clarified how multi-window applies to activities in task stack

Per bug, devs were confused about behavior of activities in a
task stack. Added a note to clarify that MW settings in the root
activity apply to all activities in stack.

See first comment for doc stage location.

bug: 27870811
Change-Id: Ifb22815e06f4040f155ec52ff5bacc81da1efbfb