math/rand: Doc fix for how many bits Seed uses
math/rand: Doc fix for how many bits Seed uses

Document the fact that the default Source uses only
the bottom 31 bits of the given seed.

Fixes #15788

Change-Id: If20d1ec44a55c793a4a0a388f84b9392c2102bd1
Reviewed-on: https://go-review.googlesource.com/23352
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

momchil-velikov/go,vsekhar/elastic-go,CAFxX/go,CAFxX/go,josharian/go,theif519/go,ImJasonH/go,cloudflare/go,golang/go,vsekhar/elastic-go,nawawi/go,goodlang/good,akutz/go,polyverse-security/go,Cofyc/go,ImJasonH/go,nawawi/go,suncycheng/go,codestation/go,theif519/go,derekmarcotte/go,derekmarcotte/go,4ad/go,theif519/go,codestation/go,cloudflare/go,4ad/go,derekmarcotte/go,polyverse-security/go,Cofyc/go,linux-on-ibm-z/go,theif519/go,momchil-velikov/go,codestation/go,cloudflare/go,linux-on-ibm-z/go,theif519/go,CAFxX/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,derekmarcotte/go,linux-on-ibm-z/go,derekmarcotte/go,cloudflare/go,goodlang/good,josharian/go,linux-on-ibm-z/go,vsekhar/elastic-go,suncycheng/go,josharian/go,polyverse-security/go,goodlang/good,codestation/go,golang/go,momchil-velikov/go,cloudflare/go,4ad/go,polyverse-security/go,suncycheng/go,Cofyc/go,codestation/go,vsekhar/elastic-go,nawawi/go,goodlang/good,josharian/go,akutz/go,christopher-henderson/Go,vsekhar/elastic-go,CAFxX/go,theif519/go,momchil-velikov/go,akutz/go,theif519/go,4ad/go,codestation/go,josharian/go,josharian/go,Cofyc/go,CAFxX/go,golang/go,ImJasonH/go,linux-on-ibm-z/go,golang/go,momchil-velikov/go,nawawi/go,CAFxX/go,goodlang/good,momchil-velikov/go,akutz/go,polyverse-security/go,ImJasonH/go,goodlang/good,linux-on-ibm-z/go,suncycheng/go,ImJasonH/go,ImJasonH/go,CAFxX/go,golang/go,ImJasonH/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,Cofyc/go,Cofyc/go,theif519/go,suncycheng/go,vsekhar/elastic-go,suncycheng/go,vsekhar/elastic-go,goodlang/good,polyverse-security/go,4ad/go,4ad/go,akutz/go,nawawi/go,4ad/go,linux-on-ibm-z/go,akutz/go,Cofyc/go,ImJasonH/go,derekmarcotte/go,Cofyc/go,suncycheng/go,josharian/go,josharian/go,vsekhar/elastic-go,momchil-velikov/go,goodlang/good,nawawi/go,golang/go,codestation/go,derekmarcotte/go,akutz/go,cloudflare/go,4ad/go,golang/go,suncycheng/go,golang/go,CAFxX/go,codestation/go,polyverse-security/go,cloudflare/go,momchil-velikov/go