net/http: clarify ConnState StateActive docs for HTTP/2

Update #13925

Change-Id: I7cd0625fad841eb0e3f364629f9bc225aa2fdce9
Reviewed-on: https://go-review.googlesource.com/18575Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/http/server.go

@@ -2016,6 +2016,11 @@ const (
 	// and doesn't fire again until the request has been
 	// handled. After the request is handled, the state
 	// transitions to StateClosed, StateHijacked, or StateIdle.
+	// For HTTP/2, StateActive fires on the transition from zero
+	// to one active request, and only transitions away once all
+	// active requests are complete. That means that ConnState
+	// can not be used to do per-request work; ConnState only notes
+	// the overall state of the connection.
 	StateActive
 
 	// StateIdle represents a connection that has finished