doc: in Go 1.8 notes, mention lack of RWMutex in contention profile
doc: in Go 1.8 notes, mention lack of RWMutex in contention profile

For #18496.

Change-Id: I50ced7c9f0fe5d9c627eef1f59a7f73be742e04c
Reviewed-on: https://go-review.googlesource.com/34831
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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