doc/go1.16: document GO386=387 and GO386=softfloat

Also add a few more TODOs as found by the relnote command.
It's an incomplete list due to #41849.

For #40700.

Change-Id: Id17a9be86d3338e1fcb281d26e7298ff26e92864
Reviewed-on: https://go-review.googlesource.com/c/go/+/260337


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>