fix various issues

- fix CI failure due to wrong usage of ticket_lifetime
- Improve document and comments

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 2a52047..944caa0 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2457,7 +2457,7 @@
  *       uint32 max_early_data_size;
  *       select ( endpoint ) {
  *            case client: ClientOnlyData;
- *            case server: uint64 ticket_creation_time_time;
+ *            case server: uint64 ticket_creation_time;
  *        };
  *     } serialized_session_tls13;
  *
@@ -2492,7 +2492,7 @@
 #endif
 
 #if defined(MBEDTLS_HAVE_TIME)
-    needed += 8; /* start_time or ticket_reception_time */
+    needed += 8; /* ticket_creation_time or ticket_reception_time */
 #endif
 
 #if defined(MBEDTLS_SSL_CLI_C)