Certificate callback doc.

diff --git a/doc/ssl/SSL_CTX_set_cert_cb.pod b/doc/ssl/SSL_CTX_set_cert_cb.pod
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+=pod
+
+=head1 NAME
+
+SSL_CTX_set_cert_cb, SSL_set_cert_cb - handle certificate callback function
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ void SSL_CTX_set_cert_cb(SSL_CTX *c, int (*cert_cb)(SSL *ssl, void *arg), void *arg);
+ void SSL_set_cert_cb(SSL *s, int (*cert_cb)(SSL *ssl, void *arg), void *arg);
+
+ int (*cert_cb)(SSL *ssl, void *arg);
+
+=head1 DESCRIPTION
+
+SSL_CTX_set_cert_cb() and SSL_set_cert_cb() sets the B<cert_cb()> callback,
+B<arg> value is pointer which is passed to the application callback.
+
+When B<cert_cb()> is NULL, no callback function is used.
+
+cert_cb() is the application defined callback. It is called before a
+certificate will be used by a client or server. The callback can then inspect
+the passed B<ssl> structure and set or clear any appropriate certificates. If
+the callback is successful it B<MUST> return 1 even if no certificates have
+been set. A zero is returned on error which will abort the handshake with a
+fatal internal error alert. A negative return value will suspend the handshake
+and the handshake function will return immediatly.
+L<SSL_get_error(3)|SSL_get_error(3)> will return SSL_ERROR_WANT_X509_LOOKUP to
+indicate, that the handshake was suspended. The next call to the handshake
+function will again lead to the call of cert_cb(). It is the job of the
+cert_cb() to store information about the state of the last call,
+if required to continue.
+
+=head1 NOTES
+
+An application will typically call SSL_use_certificate() and
+SSL_use_PrivateKey() to set the end entity certificate and private key.
+It can add intermediate and optionally the root CA certificates using
+SSL_add1_chain_cert().
+
+It might also call SSL_certs_clear() to delete any certificates associated
+with the B<SSL> object.
+
+The certificate callback functionality supercedes the (largely broken)
+functionality provided by the old client certificate callback interface.
+It is B<always> called even is a certificate is already set so the callback
+can modify or delete the existing certificate.
+
+A more advanced callback might examine the handshake parameters and set
+whatever chain is appropriate. For example a legacy client supporting only
+TLS v1.0 might receive a certificate chain signed using SHA1 whereas a
+TLS v1.2 client which advertises support for SHA256 could receive a chain
+using SHA256.
+
+Normal server sanity checks are performed on any certificates set
+by the callback. So if an EC chain is set for a curve the client does not
+support it will B<not> be used.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_use_certificate(3)|SSL_use_certificate(3)>,
+L<SSL_add1_chain_cert(3)|SSL_add1_chain_cert(3)>,
+L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)>,
+L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>
+
+=cut