Add documentation for SSL_get_server_tmp_key()

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3114)

doc/man3/SSL_get_server_tmp_key.pod

@@ -0,0 +1,43 @@
+=pod
+
+=head1 NAME
+
+SSL_get_server_tmp_key - get information about the server's temporary key used
+during a handshake
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ long SSL_get_server_tmp_key(SSL *ssl, EVP_PKEY **key);
+
+=head1 DESCRIPTION
+
+SSL_get_server_tmp_key() returns the temporary key provided by the server and
+used during key exchange. For example, if ECDHE is in use, then this represents
+the server's public ECDHE key. On success a pointer to the key is stored in
+B<*key>. It is the caller's responsibility to free this key after use using
+L<EVP_PKEY_free(3)>. This function may only be called by the client.
+
+=head1 RETURN VALUES
+
+SSL_get_server_tmp_key() returns 1 on success or 0 otherwise.
+
+=head1 NOTES
+
+This function is implemented as a macro.
+
+=head1 SEE ALSO
+
+L<ssl(7)>, L<EVP_PKEY_free(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