doc: add more Go 1.9 notes
doc: add more Go 1.9 notes

Updates #20587

Change-Id: I7effe922242db45f3ce74882d07511aaaac2f634
Reviewed-on: https://go-review.googlesource.com/45613
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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