doc/go1.16: fill in Go 1.16 release note TODOs using relnote

The additions were generated using golang.org/x/build/cmd/relnote
at CL 272907. It was modified to find previously-missed entries
by querying the Gerrit API in addition to the maintner corpus.

For #40700.
Updates #41849.

Change-Id: If575984fe40e0133ad5e8fc5411ea5063457250d
Reviewed-on: https://go-review.googlesource.com/c/go/+/272871


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>