doc/go1.14: mention new field net/http/httptest/Server.EnableHTTP2
Updates #34939
Updates #36878
Change-Id: Ifa9a17b5b16bfcfbfe1d113a2b66a63ea3a6b59c
Reviewed-on: https://go-review.googlesource.com/c/go/+/217131
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>