fixes #286 nng_pair0_open (and all others) need man page

fixes #279 consider restructuring man sections

This represents a rather significant rework, and major editing
effort, for the entire set of manual pages.

All of the pages now have a section number in their filename;
this assists in some other tooling, particularly ebook generation
as every link needs to be programmatically modified when combined
into an ebook.

Section 5 is introduced, and populated with pages for the main
types, and all options are now documented.

Numerous errors have been corrected, including rewriting certain
portions such as the header section of the surveyor protocol.

Much work has been done to facilitate index generation, although
certainly more work remains here.

Every internal link within these pages now resolves; there are no
more dead links.  (This is required to generate Kindle format books.)

1 files changed, 91 insertions, 0 deletions

diff --git a/docs/man/nng_sendmsg.3.adoc b/docs/man/nng_sendmsg.3.adoc
new file mode 100644
index 00000000..e6d653b3
--- /dev/null
+++ b/docs/man/nng_sendmsg.3.adoc
@@ -0,0 +1,91 @@
+= nng_sendmsg(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_sendmsg - send message
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/nng.h>
+
+int nng_sendmsg(nng_socket s, nng_msg *msg, int flags);
+----
+
+== DESCRIPTION
+
+The `nng_sendmsg()` sends message _msg_ using the socket _s_. 
+
+If the function returns zero, indicating it has accepted the message for
+delivery, then the _msg_ is "`owned`" by the socket _s_, and the caller
+must not make any further use of it.
+The socket will free the message when it is finished.
+
+If the function returns non-zero, then it is the caller's responsibility
+to dispose of the _msg_, which may include freeing it, sending it to
+another socket, or simply trying again later.
+
+TIP: Using this function gives access to the message structure, and may
+offer more functionality than the simpler <<nng_send.3#,`nng_send()`>> function.
+
+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 a <<nng_pub.7#,_pub_>> socket the data is broadcast, so that
+any peers who have a suitable subscription will be able to receive it using
+<<nng_recv.3#,`nng_recv()`>> or a similar function.)
+Furthermore, some protocols may not support sending (such as
+<<nng_sub.7#,_sub_>>) or may require other conditions.
+(For example, <<nng_rep.7#,_rep_>> sockets cannot normally send data, which
+are responses to requests, until they have first received a request.)
+
+The _flags_ may contain the following value:
+
+`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.
+
+
+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.7#,_pub_>> sockets, which are best-effort delivery only.
+
+== 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_msg_alloc.3#,nng_msg_alloc(3)>>,
+<<nng_recvmsg.3#,nng_recvmsg(3)>>,
+<<nng_send.3#,nng_send(3)>>,
+<<nng_strerror.3#,nng_strerror(3)>>,
+<<nng_msg.5#,nng_msg(5)>>,
+<<nng_socket.5#,nng_socket(5)>>,
+<<nng.7#,nng(7)>>