Add documentation for SSL_*_ex_data() functions

[skip ci]
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3050)

doc/man3/SSL_CTX_set_ex_data.pod

+=pod
+
+=head1 NAME
+
+SSL_CTX_get_ex_data, SSL_CTX_set_ex_data,
+SSL_get_ex_data, SSL_set_ex_data,
+SSL_SESSION_get_ex_data, SSL_SESSION_set_ex_data
+- Store and retrieve extra data from the SSL_CTX, SSL or SSL_SESSION
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ void *SSL_CTX_get_ex_data(const SSL_CTX *s, int idx);
+
+ int SSL_CTX_set_ex_data(SSL_CTX *s, int idx, void *arg);
+
+ void *SSL_get_ex_data(const SSL *s, int idx);
+
+ int SSL_set_ex_data(SSL *s, int idx, void *arg);
+
+ int SSL_SESSION_set_ex_data(SSL_SESSION *s, int idx, void *arg);
+
+ void *SSL_SESSION_get_ex_data(const SSL_SESSION *s, int idx);
+
+=head1 DESCRIPTION
+
+SSL*_set_ex_data() functions can be used to store arbitrary user data into the
+B<SSL_CTX>, B<SSL> or B<SSL_SESSION>. The user must supply a unique index
+which they can subsequently use to retrieve the data using SSL*_get_ex_data().
+
+For more detailed information see L<CRYPTO_get_ex_data(3)> and
+L<CRYPTO_set_ex_data(3)> which implement these functions and
+L<CRYPTO_get_ex_new_index(3)> for generating a unique index.
+
+=head1 RETURN VALUES
+
+The SSL*_set_ex_data() functions return 1 if the item is successfully stored
+and 0 if it is not.
+The SSL*_get_ex_data() functions return the ex_data pointer if successful,
+otherwise NULL.
+
+=head1 SEE ALSO
+
+L<CRYPTO_get_ex_data(3)>, L<CRYPTO_set_ex_data(3)>,
+L<CRYPTO_get_ex_new_index(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2017 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