go/token: add example for retrieving Position from Pos

There are few uses for the majority of the API in go/token for the
average user. The exception to this is getting the filename, line, and
column information from a token.Pos (reported and absolute. This is
straightforward but figuring out how to do it requires combing through
a lot of documentation. This example makes it more easily discoverable.

Updates #24352.

Change-Id: I0a45da6173b3dabebf42484bbbed30d9e5e20e01
Reviewed-on: https://go-review.googlesource.com/100058


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>