crypto/internal/fips140/drbg: add Counter warning

crypto/internal/fips140/drbg: add Counter warning

The support we provide is tightly scoped, and is not safe for generic
usage elsewhere in the standard library.

Change-Id: Ic38d5c4b416859ab30e2b4a3fc977ba8a2535ae8
Reviewed-on: https://go-review.googlesource.com/c/go/+/647815
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>