runtime: document that NumCPU does not change
runtime: document that NumCPU does not change

Fixes #11609.

Change-Id: I3cf64164fde28ebf739706728b84d8ef5b6dc90e
Reviewed-on: https://go-review.googlesource.com/17456
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,codestation/go,polyverse-security/go,sndnvaps/go,Samurais/go,derekmarcotte/go,roth1002/go,ganboing/go-esx,deft-code/go,kim-racktop/go,ImJasonH/go,Endika/go,kim-racktop/go,miolini/go,sndnvaps/go,derekmarcotte/go,vanloswang/go,CAFxX/go,Samurais/go,miolini/go,nawawi/go,golang/go,miolini/go,vanloswang/go,CAFxX/go,golang/go,CAFxX/go,josharian/go,fjballest/golang,codestation/go,binarycrusader/go,goodlang/good,sndnvaps/go,fjballest/golang,binarycrusader/go,derekmarcotte/go,ganboing/go-esx,goodlang/good,4ad/go,akutz/go,goodlang/good,akutz/go,odeke-em/go,roth1002/go,linux-on-ibm-z/go,vanloswang/go,CAFxX/go,Cofyc/go,christopher-henderson/Go,kim-racktop/go,suncycheng/go,cyberroadie/go,ganboing/go-esx,CAFxX/go,binarycrusader/go,golang/go,momchil-velikov/go,vsekhar/elastic-go,Cofyc/go,fjballest/golang,roth1002/go,cyberroadie/go,polyverse-security/go,nawawi/go,deft-code/go,cyberroadie/go,ImJasonH/go,theif519/go,sndnvaps/go,vsekhar/elastic-go,vsekhar/elastic-go,cloudflare/go,Endika/go,nawawi/go,cyberroadie/go,mwhudson/go,josharian/go,codestation/go,suncycheng/go,ImJasonH/go,roth1002/go,suncycheng/go,4ad/go,theif519/go,theif519/go,vsekhar/elastic-go,binarycrusader/go,4ad/go,sndnvaps/go,akutz/go,akutz/go,kim-racktop/go,nawawi/go,Endika/go,theif519/go,josharian/go,odeke-em/go,Cofyc/go,josharian/go,goodlang/good,CAFxX/go,deft-code/go,cloudflare/go,codestation/go,kim-racktop/go,linux-on-ibm-z/go,kim-racktop/go,codestation/go,4ad/go,cyberroadie/go,ganboing/go-esx,Cofyc/go,sndnvaps/go,4ad/go,roth1002/go,derekmarcotte/go,cyberroadie/go,codestation/go,Cofyc/go,vsekhar/elastic-go,suncycheng/go,codestation/go,suncycheng/go,ganboing/go-esx,ImJasonH/go,vanloswang/go,mwhudson/go,cloudflare/go,Cofyc/go,odeke-em/go,cloudflare/go,suncycheng/go,mwhudson/go,miolini/go,goodlang/good,Samurais/go,ImJasonH/go,odeke-em/go,fjballest/golang,binarycrusader/go,linux-on-ibm-z/go,theif519/go,cyberroadie/go,binarycrusader/go,Samurais/go,mwhudson/go,odeke-em/go,momchil-velikov/go,polyverse-security/go,linux-on-ibm-z/go,deft-code/go,roth1002/go,codestation/go,vsekhar/elastic-go,roth1002/go,4ad/go,ImJasonH/go,nawawi/go,vanloswang/go,binarycrusader/go,Samurais/go,ganboing/go-esx,ganboing/go-esx,theif519/go,josharian/go,Cofyc/go,odeke-em/go,theif519/go,mwhudson/go,cloudflare/go,ImJasonH/go,golang/go,akutz/go,cloudflare/go,deft-code/go,polyverse-security/go,kim-racktop/go,derekmarcotte/go,fjballest/golang,deft-code/go,Endika/go,linux-on-ibm-z/go,linux-on-ibm-z/go,goodlang/good,miolini/go,goodlang/good,derekmarcotte/go,4ad/go,mwhudson/go,theif519/go,polyverse-security/go,Samurais/go,golang/go,polyverse-security/go,sndnvaps/go,vsekhar/elastic-go,goodlang/good,cyberroadie/go,josharian/go,fjballest/golang,linux-on-ibm-z/go,miolini/go,mwhudson/go,deft-code/go,golang/go,binarycrusader/go,vanloswang/go,Samurais/go,josharian/go,momchil-velikov/go,Endika/go,Endika/go,akutz/go,deft-code/go,cloudflare/go,momchil-velikov/go,kim-racktop/go,odeke-em/go,polyverse-security/go,fjballest/golang,akutz/go,derekmarcotte/go,linux-on-ibm-z/go,ImJasonH/go,Samurais/go,roth1002/go,polyverse-security/go,fjballest/golang,CAFxX/go,golang/go,CAFxX/go,vanloswang/go,ganboing/go-esx,golang/go,momchil-velikov/go,miolini/go,suncycheng/go,odeke-em/go,mwhudson/go,momchil-velikov/go,nawawi/go,derekmarcotte/go,akutz/go,Endika/go,Cofyc/go,suncycheng/go,miolini/go,sndnvaps/go,vsekhar/elastic-go,cloudflare/go,vanloswang/go,josharian/go,4ad/go,momchil-velikov/go,nawawi/go,momchil-velikov/go,Endika/go