net/http: document zero value of Protocols

For #67814

Change-Id: I182e9c7e720493adb9d2384336e757dace818525
Reviewed-on: https://go-review.googlesource.com/c/go/+/635335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/net/http/http.go

@@ -17,6 +17,7 @@ import (
 )
 
 // Protocols is a set of HTTP protocols.
+// The zero value is an empty set of protocols.
 //
 // The supported protocols are:
 //