Add nng_sendmsg and nng_recvmsg manual pages.

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diff --git a/docs/man/nng_recvmsg.adoc b/docs/man/nng_recvmsg.adoc
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+= nng_recvmsg(3)
+//
+// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
+// Copyright 2018 Capitar IT Group BV <info@capitar.com>
+//
+// This document is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+== NAME
+
+nng_recvmsg - recv message
+
+== SYNOPSIS
+
+[source, c]
+-----------
+#include <nng/nng.h>
+
+int nng_recvmsg(nng_socket s, nng_msg **msgp, int flags);
+-----------
+
+== DESCRIPTION
+
+The `nng_recvmsg()` receives a message on socket _s_, storing the
+received message at the location pointed to by _msgp_.
+
+TIP: Using this function gives access to the message structure, and thus may
+offer more functionality than the simpler <<nng_recv#,nng_recv(3)>> function.
+
+The _flags_ may contain the following value:
+
+`NNG_FLAG_NONBLOCK`::
+  The function returns immediately, even if no message is available.  Without
+  this flag, the function will wait until a message is received by the socket
+  _s_, or any configured timer expires.
+
+NOTE: The semantics of what receiving a message means vary from protocol to
+protocol, so examination of the protocol documentation is encouraged.  (For
+example, with an <<nng_req#,nng_req(7)>> socket a message may only be received
+after a request has been sent, and an <<nng_sub#,nng_sub(7)>> socket
+may only receive messages corresponding to topics to which it has subscribed.)
+Furthermore, some protocols may not support receiving data at all, such as
+<<nng_pub#,nng_pub(7)>>.
+
+== RETURN VALUES
+
+This function returns 0 on success, and non-zero otherwise.
+
+== ERRORS
+
+`NNG_EAGAIN`:: The socket _s_ cannot accept data for sending.
+`NNG_ECLOSED`:: The socket _s_ is not open.
+`NNG_EINVAL`:: An invalid set of _flags_ was specified.
+`NNG_ENOMEM`:: Insufficient memory is available.
+`NNG_ENOTSUP`:: The protocol for socket _s_ does not support receiving.
+`NNG_ESTATE`:: The socket _s_ cannot receive data in this state.
+
+== SEE ALSO
+
+<<nng_msg_free#,nng_msg_free(3)>>,
+<<nng_recv#,nng_recv(3)>>,
+<<nng_sendmsg#,nng_sendmsg(3)>>,
+<<nng_strerror#,nng_strerror(3)>>,
+<<nng#,nng(7)>>