net/http/httptrace: refer http.Client users to the blog post
net/http/httptrace: refer http.Client users to the blog post

Fixes #17152.

Change-Id: I4dd5e505c65f3efe736e46d3781cccf31d7f574f
Reviewed-on: https://go-review.googlesource.com/32117
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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