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.

Author: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Discussion: https://postgr.es/m/CAKZiRmwBcALLrDgCyEhHP1enUxtPMjyNM_d1A2Lng3_6Rf4Qfw%40mail.gmail.com

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 might not be sent at the required interval.  In
+        extreme cases, this can lead to a prolonged risk of not removing dead
+        rows on the primary for extended periods, as the feedback mechanism is
+        based on timestamps.
+       </para>
       </listitem>
      </varlistentry>