_content/doc: add Go 1.16 release notes

Copied from Go commit 094048b93845c08e0f2db8639764f9564e64835b,
deleted there in CL 295209.

Include the fix from CL 292471: remove a resolved TODO comment.

Updates golang/go#40700.

Change-Id: I0c7a9b3855d4205783ca631aaeb64b6b52dde0c5
Reviewed-on: https://go-review.googlesource.com/c/website/+/295249
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: dmitshur