correcting --expire_logs_days description in mysqld --help

sql/mysqld.cc

@@ -4758,7 +4758,8 @@ replicating a LOAD DATA INFILE command.",
     (gptr*) &delayed_queue_size, (gptr*) &delayed_queue_size, 0, GET_ULONG,
     REQUIRED_ARG, DELAYED_QUEUE_SIZE, 1, ~0L, 0, 1, 0},
   {"expire_logs_days", OPT_EXPIRE_LOGS_DAYS,
-   "Binary logs will be rotated after expire-log-days days ",
+   "If non-zero, binary logs will be purged after expire_logs_days "
+   "days; possible purges happen at startup and at binary log rotation.",
    (gptr*) &expire_logs_days,
    (gptr*) &expire_logs_days, 0, GET_ULONG,
    REQUIRED_ARG, 0, 0, 99, 0, 1, 0},