Doc: Update the documentation for spilled transaction statistics.

Reported-by: Sawada Masahiko
Author: Sawada Masahiko, Amit Kapila
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CA+fd4k4vNg7dRO5ECHdtQXXf1=Q4M98pfLW0dU7BKD8h79pkqA@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -2518,6 +2518,12 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
    mechanism will simply display NULL lag.
   </para>
 
+  <para>
+   Tracking of spilled transactions works only for logical replication.  In
+   physical replication, the tracking mechanism will display 0 for spilled
+   statistics.
+  </para>
+
   <note>
    <para>
     The reported lag times are not predictions of how long it will take for