Add documentation for DTLS_get_data_mtu()

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

doc/man3/DTLS_get_data_mtu.pod

@@ -0,0 +1,36 @@
+=pod
+
+=head1 NAME
+
+DTLS_get_data_mtu - Get maximum data payload size
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ size_t DTLS_get_data_mtu(const SSL *ssl);
+
+=head1 DESCRIPTION
+
+This function obtains the maximum data payload size for the established
+DTLS connection B<ssl>, based on the DTLS record MTU and the overhead
+of the DTLS record header, encryption and authentication currently in use.
+
+=head1 RETURN VALUES
+
+Returns the maximum data payload size on success, or 0 on failure.
+
+=head1 HISTORY
+
+This function was added in OpenSSL 1.1.1
+
+=head1 COPYRIGHT
+
+Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut