doc/go1.19: add more TODOs from updated relnote

CL 410244 changes relnote to look for api file changes as well
as references to proposal issues, finding various things that
were missing from the release notes.

This CL adds the TODOs that the updated relnote found.

For #51400.

Change-Id: I512a9b8f1349a6c68c8a6979f55a07964d630175
Reviewed-on: https://go-review.googlesource.com/c/go/+/410361
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: rsc