math/rand: clarify documentation for Seed example
math/rand: clarify documentation for Seed example

Fixes #25325

Change-Id: I101641be99a820722edb7272918e04e8d2e1646c
Reviewed-on: https://go-review.googlesource.com/112775
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

cloudflare/go,josharian/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,akutz/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,golang/go,akutz/go,josharian/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,golang/go,josharian/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,akutz/go,josharian/go,golang/go,nawawi/go