DOC: unreliable sockpair@ on macOS

DOC: unreliable sockpair@ on macOS

We discovered that the sockpair@ protocol is unreliable in macOS, this
is the same problem that we fixed in d7f6819. But it's not possible to
implement a acknowledgment once the socket are in non-blocking mode.

The problem was discovered in issue #3045.

Must be backported in every stable versions.

(cherry picked from commit 8a456399dba70c9880394a7e85f9523feb1505c5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 0746af209c3600a908b141f213c8ba7bb807840a)
[cf: ctx adjt]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 16ab3761721b11803d6b5df38a487a8bf48a8b49)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>