Urge users of math/rand to consider using crypto/rand when doing
security-sensitive work.

Related to issue #11871. While we haven't reached consensus on how
to make the package inherently safer, everyone agrees that the docs
for math/rand can be improved.

Change-Id: I576a312e51b2a3445691da6b277c7b4717173197
Reviewed-on: https://go-review.googlesource.com/12900Reviewed-by: Rob Pike <r@golang.org>