net/http: mention RegisterOnShutdown in Server.Shutdown docs
net/http: mention RegisterOnShutdown in Server.Shutdown docs

Closes #21637

Change-Id: Icc3528572ea2a25e62757cc8fbbb9c3fa96a78b2
Reviewed-on: https://go-review.googlesource.com/59350
Reviewed-by: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>

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