net/http: document that ServeMux.Handler can also synthetize a 405

net/http: document that ServeMux.Handler can also synthetize a 405

Also, fix a minor typo in ServeMux.Handle and ServeMux.HandleFunc.

Change-Id: I6a6a46565719104cb8f2484daf0e39f35b55a078
Reviewed-on: https://go-review.googlesource.com/c/go/+/675835
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>