Improve documents

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 1d5f428..ce25949 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -4137,12 +4137,19 @@
     defined(MBEDTLS_SSL_SRV_C) && \
     defined(MBEDTLS_SSL_PROTO_TLS1_3)
 /**
- * \brief          Number of NewSessionTicket messages for the server to send
- *                 after handshake completion.
- *                 (Default: MBEDTLS_SSL_TLS1_3_DEFAULT_NEW_SESSION_TICKETS)
+* \brief    Number of NewSessionTicket messages for the server to send
+ *           after handshake completion.
+ * \note    The default value is
+ *          MBEDTLS_SSL_TLS1_3_DEFAULT_NEW_SESSION_TICKETS.
+ * \note    In case of a session resumption, this setting only partially
+ *          apply. At most one ticket is sent in that case to just review the
+ *          pool of tickets of the client. The rationale is to avoid the
+ *          number of tickets on the server to become rapidly out of
+ *          control when the server has the same configuration for all its
+ *          connection instances.
  *
- * \param conf     SSL configuration
- * \param num_tickets   Number of NewSessionTicket.
+ * \param conf    SSL configuration
+ * \param num_tickets    Number of NewSessionTicket.
  *
  */
 void mbedtls_ssl_conf_new_session_tickets( mbedtls_ssl_config *conf,