cmd/doc: treat any non-empty GOMOD as module mode

Previously, we were looking for the string go.mod specifically, but
the module-mode-outside-a-module logic added in CL 148517 sets GOMOD
to os.DevNull

Updates #28992

Change-Id: I62a4baaa911a495350294d78bae96be3fe4866cb
Reviewed-on: https://go-review.googlesource.com/c/151617


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>