doc/next: document that crypto/rand.Read never fails

For #66821

Change-Id: Id9b640a57b9d4d1f9114769f607480b14961e7b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/632036
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: ianlancetaylor

doc/next/6-stdlib/99-minor/crypto/rand/66821.md

@@ -0,0 +1,7 @@
+The [Read] function, and the `Read` method of [Reader], are now
+defined to never fail.
+They will always return `nil` as the `error` result.
+If something somehow goes wrong while reading random numbers,
+the program will irrecoverably crash.
+This change was made because all supported systems now provide
+sources of random bytes that never fail.