net/http: document restrictions on ETag as expected by ServeContent
net/http: document restrictions on ETag as expected by ServeContent

Fixes #18054

Change-Id: I6773943a95b92eebd7e347f8f7a80843b4827243
Reviewed-on: https://go-review.googlesource.com/33630
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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