net/http: document internal error errServerClosedIdle more
net/http: document internal error errServerClosedIdle more

Updates #19943

Change-Id: Iea249be51a7af3264bee9ee2b28dbd91043275fc
Reviewed-on: https://go-review.googlesource.com/86375
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,nawawi/go,Cofyc/go,golang/go,codestation/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,Cofyc/go,cloudflare/go,codestation/go,CAFxX/go,josharian/go,nawawi/go,akutz/go,josharian/go,josharian/go,josharian/go,golang/go,codestation/go,CAFxX/go,josharian/go,akutz/go,christopher-henderson/Go,nawawi/go,nawawi/go,CAFxX/go,codestation/go,akutz/go,Cofyc/go,akutz/go,CAFxX/go,Cofyc/go,CAFxX/go,golang/go,codestation/go,cloudflare/go,cloudflare/go,cloudflare/go,Cofyc/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,codestation/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,josharian/go,akutz/go,Cofyc/go,cloudflare/go,Cofyc/go,codestation/go,cloudflare/go,golang/go,nawawi/go,codestation/go,golang/go,Cofyc/go,nawawi/go,golang/go,nawawi/go