doc/go1.19: document Resolver.PreferGo

doc/go1.19: document Resolver.PreferGo

Updates #51400

Change-Id: I61733574362d4cf3cb65122bd13361e5c0f6728c
Reviewed-on: https://go-review.googlesource.com/c/go/+/410375
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>