net/http: clarify ServeMux.Handler behavior

net/http: clarify ServeMux.Handler behavior

Explain that ServeMux.Handler doesn't populate the request with
matches.

Fixes #69623.

Change-Id: If625b3f8e8f4e54b05e1d9a86e8c471045e77763
Reviewed-on: https://go-review.googlesource.com/c/go/+/674095
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Chressie Himpel <chressie@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>