syscall: document that Exec wraps execve(2)
syscall: document that Exec wraps execve(2)

Change-Id: I611511434f37c75f77c22f61f469108243bc0101
Reviewed-on: https://go-review.googlesource.com/29121
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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