Improve documentation about SSL ticket encryption
diff --git a/include/mbedtls/ssl_ticket.h b/include/mbedtls/ssl_ticket.h
index caa8380..abce61e 100644
--- a/include/mbedtls/ssl_ticket.h
+++ b/include/mbedtls/ssl_ticket.h
@@ -87,14 +87,19 @@
* \param ctx Context to be set up
* \param f_rng RNG callback function
* \param p_rng RNG callback context
- * \param cipher AEAD cipher to use for ticket protection, eg
- * MBEDTLS_CIPHER_AES_256_GCM or MBEDTLS_CIPHER_AES_256_CCM.
+ * \param cipher AEAD cipher to use for ticket protection.
+ * Recommended value: MBEDTLS_CIPHER_AES_256_GCM.
* \param lifetime Tickets lifetime in seconds
+ * Recommended value: 86400 (one day).
*
* \note It is highly recommended to select a cipher that is at
* least as strong as the the strongest ciphersuite
* supported. Usually that means a 256-bit key.
*
+ * \note The lifetime of the keys is twice the lifetime of tickets.
+ * It is recommended to pick a reasonnable lifetime so as not
+ * to negate the benefits of forward secrecy.
+ *
* \return 0 if successful,
* or a specific MBEDTLS_ERR_XXX error code
*/