The current documentation of the WriteTo function is very poor and it
is difficult to deduce how to use it correctly. A good example will
make things much easier.

Fixes #25456

Change-Id: Ibf0c0e153afae8f3e0d7d765d0dc9bcbfd69bfb1
Reviewed-on: https://go-review.googlesource.com/113775
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>