message: add documentation and examples Change-Id: I5b96c78428b1650d13f56db02a7afed295c7298e Reviewed-on: https://go-review.googlesource.com/59030 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>