crypt/rand: update docs for Linux

Update the docs to explain the code added in
commit 67e1d400.

Fixes #11831.

Change-Id: I8fe72e449507847c4bd9d77de40947ded7f2ff9d
Reviewed-on: https://go-review.googlesource.com/12515Reviewed-by: Dave Cheney <dave@cheney.net>

src/crypto/rand/rand.go

@@ -10,7 +10,9 @@ import "io"
 
 // Reader is a global, shared instance of a cryptographically
 // strong pseudo-random generator.
+//
 // On Unix-like systems, Reader reads from /dev/urandom.
+// On Linux, Reader uses getrandom(2) if available, /dev/urandom otherwise.
 // On Windows systems, Reader uses the CryptGenRandom API.
 var Reader io.Reader