net: add Example for Unix dialer with context

Showing users how to accomplish this more easily is a compromise
between doing nothing and adding a new API to the net package.

Fixes #38506.

Change-Id: I43f831cf94951c987cf3c8c1aa55f0012ee8034e
Reviewed-on: https://go-review.googlesource.com/c/go/+/263417


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>