Add nng_send and nng_recv man pages.

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diff --git a/docs/nng_send.adoc b/docs/nng_send.adoc
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+= nng_send(3)
+:doctype: manpage
+:manmanual: nng
+:mansource: nng
+:manvolnum: 3
+:copyright: Copyright 2018 mailto:info@staysail.tech[Staysail Systems, Inc.] + \
+            Copyright 2018 mailto:info@capitar.com[Capitar IT Group BV] + \
+            {blank} + \
+            This document is supplied under the terms of the \
+            https://opensource.org/licenses/MIT[MIT License].
+
+== NAME
+
+nng_send - send data
+
+== SYNOPSIS
+
+[source, c]
+-----------
+#include <nng/nng.h>
+
+int nng_send(nng_socket s, void *data, size_t size, int flags);
+-----------
+
+== DESCRIPTION
+
+The `nng_send()` sends a message containing the _data_ of length _size_
+using the socket _s_.
+
+NOTE: The semantics of what sending a message means vary from protocol to
+protocol, so examination of the protocol documentation is encouraged.  (For
+example, with an <<nng_pub#,nng_pub(3)>> socket the data is broadcast, so that
+any peers who have a suitable subscription will be able to receive it using
+<<nng_recv#,nng_recv(3)>> or a similar function.)  Furthermore, some protocols
+may not support sending data (such as <<nng_sub#,nng_sub(3)>>) or may
+require other conditions.  (For example, <<nng_rep#,nng_rep(3)>> sockets
+cannot normally send data, which are responses to requests, until they have
+first received a request.)
+
+The _flags_ may contain either of (or neither of) the following values:
+
+`NNG_FLAG_NONBLOCK`::
+    The function returns immediately, regardless of whether
+    the socket is able to accept the data or not.  If the socket is unable
+    to accept the data (such as if backpressure exists because the peers
+    are consuming messages too slowly, or no peer is present), then the
+    function will return with `NNG_EAGAIN`.  If this flag is not specified,
+    then the function will block if such a condition exists.
+
+`NNG_FLAG_ALLOC`::
+    The _data_ was allocated using <<nng_alloc#,nng_alloc(3)>>, or was obtained
+    from a call to <<nng_recv#,nng_recv(3)>> with the `NNG_FLAG_ALLOC` flag.
+    If this function returns success, then the _data_ is "owned" by the
+    function, and it will assume responsibility for calling
+    <<nng_free#,nng_free(3)>> when it is no longer needed.  In the absence
+    of this flag, the _data_ is copied by the implementation before the
+    function returns to the caller.
+
+TIP: The `NNG_FLAG_ALLOC` flag can be used to reduce data copies, thereby
+increasing performance.
+
+NOTE: Regardless of the presence or absence of `NNG_FLAG_NONBLOCK`, there may
+be queues between the sender and the receiver.  Furthermore, there is no
+guarantee that the message has actually been delivered.  Finally, with some
+protocols, the semantic is implictly `NNG_FLAG_NONBLOCK`, such as with
+<<nng_pub#,nng_pub(3)>> sockets, which are best-effort delivery only.
+
+WARNING: When using `NNG_FLAG_ALLOC`, it is important that the value of _size_
+match the actual allocated size of the data.  Using an incorrect size results
+in unspecified behavior, which may include heap corruption, program crashes,
+or transdimensional mutation of the program's author.
+
+== 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_EMSGSIZE`:: The value of _size_ is too large.
+`NNG_ENOMEM`:: Insufficient memory is available.
+`NNG_ENOTSUP`:: The protocol for socket _s_ does not support sending.
+`NNG_ESTATE`:: The socket _s_ cannot send data in this state.
+
+== SEE ALSO
+
+<<nng_alloc#,nng_alloc(3)>>,
+<<nng_free#,nng_free(3)>>,
+<<nng_recv,nng_recv(3)>>,
+<<nng_sendmsg#,nng_sendmsg(3)>>,
+<<nng_strerror#,nng_strerror(3)>>,
+<<nng#,nng(7)>>
+
+== COPYRIGHT
+
+{copyright}