Added clarifying comments to event distribution code

When a slave transitions from catchup mode to up-to-date mode, it is possible
that events are being distributed and the slave appears to be lagging behind.
The added comment explain why it happens and how it is handled to make the code
clearer.

server/modules/routing/binlog/blr_master.c

@@ -1927,6 +1927,10 @@ blr_distribute_binlog_record(ROUTER_INSTANCE *router, REP_HEADER *hdr, uint8_t *
                               slave->binlogfile, slave->binlog_pos,
                               hdr->next_pos - hdr->event_size);
                 }
+                /* If none of the above conditions were met, a slave in catchup
+                 * mode transitioned into up-to-date mode while we were
+                 * distributing events. The slave needs to be forced into
+                 * catchup mode since it isn't up to date anymore. */
             }
             else if (rotate)
             {