doc/go1.8: document that CGO_ENABLED is sticky
doc/go1.8: document that CGO_ENABLED is sticky

Fixes #18363.

Change-Id: Ifc98506d33a6753cd7db8e505cf86d5626fbbad0
Reviewed-on: https://go-review.googlesource.com/34596
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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