doc: inserted TODOs into 9-todo.md

Two known unnecessary/done ones have been removed, the
rest are as is.

Change-Id: I859efee292e06cd4877a814280950a47fac5bf43
Reviewed-on: https://go-review.googlesource.com/c/go/+/784480
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>