Improve the documentation of some TRX_RSEG fields

Author: dr-m

storage/innobase/include/trx0rseg.h

@@ -212,12 +212,12 @@ struct trx_rseg_t {
 
 /* Transaction rollback segment header */
 /*-------------------------------------------------------------*/
-#defineTRX_RSEG_FORMAT0/* -2 = pre-MariaDB 10.3.5 format;
-0=MariaDB 10.3.5 or later */
-#defineTRX_RSEG_HISTORY_SIZE4/* Number of file pages occupied
-by the logs in the history list */
-#defineTRX_RSEG_HISTORY8/* The update undo logs for committed
-transactions */
+/** 0xfffffffe = pre-MariaDB 10.3.5 format; 0=MariaDB 10.3.5 or later */
+#defineTRX_RSEG_FORMAT0
+/** Number of pages in the TRX_RSEG_HISTORY list */
+#defineTRX_RSEG_HISTORY_SIZE4
+/** Committed transaction logs that have not been purged yet */
+#defineTRX_RSEG_HISTORY8
 #defineTRX_RSEG_FSEG_HEADER(8 + FLST_BASE_NODE_SIZE)
 /* Header for the file segment where
 this page is placed */