doc/go1.9: GC performance release notes
doc/go1.9: GC performance release notes

Change-Id: I361587ba0ddffb5ee4a3d1bdb6219710a30da197
Reviewed-on: https://go-review.googlesource.com/45132
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

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