This clarifies these functions' behavior when the non-control data is
empty but there is a control data present.
It also makes clear the difference between Sendmsg, SendmsgN, and
SendmsgBuffers, and between Recvmsg and RecvmsgBuffers.
Fixesgolang/go#56911
Change-Id: I1e35659e66e493c192af7f5a426a316f6052475c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/456816
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
