doc: Mention clock synchronization recommendation for hot_standby_feedback

hot_standby_feedback mechanics assume that clocks are synchronized, but it was not clear from documentation.

Discussion: https://postgr.es/m/CAKZiRmwBcALLrDgCyEhHP1enUxtPMjyNM_d1A2Lng3_6Rf4Qfw%40mail.gmail.com

Author: jakubwartakEDB

doc/src/sgml/config.sgml

@@ -5068,6 +5068,13 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         until it eventually reaches the primary.  Standbys make no other use
         of feedback they receive other than to pass upstream.
        </para>
+       <para>
+        Note that if the clock on standby is moved ahead or backward, the
+        feedback message may not be sent at the required interval. In extreme
+        cases this can lead to prolonged risk of not removing dead rows on
+        primary for extended periods as the feedback mechanism is based on
+        timestamp.
+       </para>
       </listitem>
      </varlistentry>