docs: PIP-related fix to N preview API overview doc

In api-overview, we removed the short section describing PIP
(because it's already mentioned in the multi-window section).
However, we still refer to it in the multi-window section by
saying "See below for more information". This CL just removes
that statement.

Staged preview URL in comment in CL

Bug: 27569802
Change-Id: Iab6f5da3af7b13eb9dc93e5f8fa786129760a5a1