doc: update godebug doc to clarify godebug directive in workspace
Make it clear that we only use godebug directives in the go.work, and
that we don't use those in go.mod, when we're in a workspace.
Fixes #72109
Change-Id: I648bfa4dd9b3ca0ac299c0a890843d41fe1ac7f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/655158
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>