net/http: update HTTP/2 documentation to reference new config features

net/http: update HTTP/2 documentation to reference new config features

Update the package docs to point users at the modern HTTP/2
configuration APIs.

Mention in the TLSNextProto documentation that this field is
superseded by the Protocols field for most user-facing purposes.

Change-Id: I30cd9a85a27e1174338f0d6b887f98c28eac5b5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/709797
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>