doc: update go1.8.html
doc: update go1.8.html

Made many minor changes so that the document is consistent with itself.

Some more noticeable changes:
* CL/34141: Revert "testing: add T.Context method"
* CL/33630: net/http: document restrictions on ETag as expected by ServeContent

Change-Id: I39ae5e55c56e374895c115e6852998c940beae35
Reviewed-on: https://go-review.googlesource.com/34243
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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