math/rand: add concurrency warning to overview comment
math/rand: add concurrency warning to overview comment

Change-Id: I52efa7aa72a23256e5ca56470ffeba975ed8f739
Reviewed-on: https://go-review.googlesource.com/48760
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

polyverse-security/go,akutz/go,vsekhar/elastic-go,nawawi/go,cloudflare/go,cloudflare/go,polyverse-security/go,nawawi/go,josharian/go,akutz/go,josharian/go,CAFxX/go,polyverse-security/go,Cofyc/go,codestation/go,cloudflare/go,CAFxX/go,polyverse-security/go,codestation/go,christopher-henderson/Go,akutz/go,polyverse-security/go,akutz/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,golang/go,josharian/go,cloudflare/go,Cofyc/go,akutz/go,josharian/go,akutz/go,vsekhar/elastic-go,golang/go,CAFxX/go,nawawi/go,Cofyc/go,golang/go,vsekhar/elastic-go,CAFxX/go,akutz/go,vsekhar/elastic-go,josharian/go,codestation/go,nawawi/go,akutz/go,vsekhar/elastic-go,codestation/go,CAFxX/go,nawawi/go,codestation/go,polyverse-security/go,nawawi/go,Cofyc/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,vsekhar/elastic-go,golang/go,polyverse-security/go,Cofyc/go,nawawi/go,codestation/go,cloudflare/go,Cofyc/go,golang/go,CAFxX/go,golang/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,josharian/go,josharian/go,cloudflare/go,Cofyc/go,codestation/go,codestation/go,golang/go,nawawi/go