spec: add example for method value in case of embedded method

So it's clear to the reader that if "M" is a promoted method from
embedded field "T", then "x.M" will be expanded to "x.T.M" during the
evaluation of the method value.

Fixes #47863

Change-Id: Id3b82127a2054584b6842c487f6e15c3102dc9fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/344209


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>