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 <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

Diff for: src/crypto/internal/fips140/drbg/rand.go

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package drbg provides cryptographically secure random bytes
+// usable by FIPS code. In FIPS mode it uses an SP 800-90A Rev. 1
+// Deterministic Random Bit Generator (DRBG). Otherwise,
+// it uses the operating system's random number generator.
 package drbg
 
 import (