docs: Continuing work on the N Preview multi-window docs

Work on multi-window doc continues here from the previous CL
(http://ag/874065/) which we submitted in the run-up to Preview 1.
Includes new note about Drag and Drop being extended for multi-window.

See first comment for doc stage location.

bug: 26558855
Change-Id: I0e71aec894e8917b0df4e586a954127c91d2d1fb