doc/go1.16: more release notes

Fixes #42899 (flag).
Fixes #42900 (io).
Fixes #42901 (log).
Fixes #42902 (log/syslog).
Fixes #42903 (mime/multipart).
Fixes #42904 (net).
Fixes #42905 (net/http).
Fixes #42906 (net/http/httputil).
Fixes #42907 (net/smtp).
Fixes #42909 (os/signal).
Fixes #42913 (syscall).

Change-Id: Id09f038751d61fe0f1ff57b525e49473dd75c95f
Reviewed-on: https://go-review.googlesource.com/c/go/+/275113


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>