Document the split-screen action

The split-screen behavior is changed, so systemui doesn't
support this action with a single activity. We don't handle
this action by default from sc-v2. If SystemUI has any behavior
changed, they could register this action to support it.

Bug: 207056257
Test: presubmit pass
Change-Id: If569eff1ac52601e6b88324d7b47630a53e198f2