crypto/internal/fips140/drbg: add package comment

crypto/internal/fips140/drbg: add package comment

For #69536

Change-Id: I70d2ed10555fed244f08c898899399e3032e17b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/640597
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>