doc: pre-announce removal of gotypesalias and asynctimerchan GODEBUG flags
This announcement follows the policy outlined in #76163.
For #76472.
Change-Id: I4299e7f474e314810883dc4f50be6afba8c3e3e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/726020
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>