ReadMsgUnix and WriteMsgUnix both will read/write 1 byte from/to the
socket if they were given no buffer to read/write, to avoid a common
pitfall in out of band operations (they will usually block
indefinitely if there's no actual data to read).

This patch adds a note about this behaviour in their documentation, so
users can be aware of it.

Change-Id: I751f0e12bb4d80311e94ea8de023595c5d40ec3e
Reviewed-on: https://go-review.googlesource.com/29180
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>