The previous documentation purported to convert underlying strings to
[]byte, which it did not do. This adds support for underlying bool,
string, and []byte, which convert directly to their underlying type.

Fixes #15174.

Change-Id: I7fc4e2520577f097a48f39c9ff6c8160fdfb7be4
Reviewed-on: https://go-review.googlesource.com/27812Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>