doc/go1.16: address some remaining high-level TODOs

The tools section TODO can be removed since the tools section looks
complete by now.

All TODOs in the minor changes to the library section have been done,
so the top-level TODO is resolved. Delete it.

The currently highlighted entries under Core library section look
good. It's worth reviewing this further based on feedback from Go
1.16 pre-releases, so keep the TODO but make it non-user-visible
to unblock Go 1.16 Beta 1.

For #40700.

Change-Id: Ie72661bd457b0a93ef92e1bfc0844072f3b618a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/277212


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