Add & document msg_capacity, msg_reserve (#1458)

* Add & document msg_capacity, msg_reserve

* reserve/capacity code style

* Documentation references to reserve/capacity

1 files changed, 46 insertions, 0 deletions

diff --git a/docs/man/nng_msg_capacity.3.adoc b/docs/man/nng_msg_capacity.3.adoc
new file mode 100644
index 00000000..ffc7f5d3
--- /dev/null
+++ b/docs/man/nng_msg_capacity.3.adoc
@@ -0,0 +1,46 @@
+= nng_msg_capacity(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_msg_capacity - return message body length
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/nng.h>
+
+size_t nng_msg_capacity(nng_msg *msg);
+----
+
+== DESCRIPTION
+
+The `nng_msg_capacity()` returns the storage allocated for the body of message _msg_.
+The capacity includes the current contents of the message and free space after it.
+The message body may grow to capacity without performing any further allocations.
+
+== RETURN VALUES
+
+Allocated capacity for message body.
+
+== ERRORS
+
+None.
+
+== SEE ALSO
+
+[.text-left]
+xref:nng_msg_reserve.3.adoc[nng_msg_reserve(3)],
+xref:nng_msg_alloc.3.adoc[nng_msg_alloc(3)],
+xref:nng_msg_body.3.adoc[nng_msg_body(3)],
+xref:nng_msg.5.adoc[nng_msg(5)],
+xref:nng.7.adoc[nng(7)]