Update ticket callback docs.

(cherry picked from commit a23a6e85d8)

doc/ssl/SSL_CTX_set_tlsext_ticket_key_cb.pod

@@ -49,8 +49,10 @@ the callback function will be called with I<enc> equal to 1. The OpenSSL
 library expects that the function will set an arbitary I<name>, initialize
 I<iv>, and set the cipher context I<ctx> and the hash context I<hctx>.
 
-The I<name> is only 16 characters long. The I<iv> is of length
-L<EVP_MAX_IV_LENGTH> defined in B<evp.h>.
+The I<name> is 16 characters long and is used as a key identifier.
+
+The I<iv> length is the length of the IV of the corresponding cipher. The
+maximum IV length is L<EVP_MAX_IV_LENGTH> bytes defined in B<evp.h>.
 
 The initialization vector I<iv> should be a random value. The cipher context 
 I<ctx> should use the initialisation vector I<iv>. The cipher context can be 
@@ -110,6 +112,17 @@ an all other negotiated state information encrypted within the ticket. In a
 resumed session the applications will have all this state information available
 exactly as if a full negiotation had occured.
 
+If an attacker can obtain the key used to encrypt a session ticket, they can
+obtain the master secret for any ticket using that key and decrypt any traffic
+using that session: even if the ciphersuite supports forward secrecy. As
+a result applications may wish to use multiple keys and avoid using long term
+keys stored in files.
+
+Applications can use longer keys to maintain a consistent level of security.
+For example if a ciphersuite uses 256 bit ciphers but only a 128 bit ticket key
+the overall security is only 128 bits because breaking the ticket key will
+enable an attacker to obtain the session keys.
+
 =head1 EXAMPLES
 
 Reference Implemention: