doc: warn pg_stat_reset() can cause vacuum/analyze problems

The fix is to run ANALYZE.

Discussion: https://postgr.es/m/YzRr+ys98UzVQJvK@momjian.us,
   https://postgr.es/m/flat/CAKJS1f8DTbCHf9gedU0He6ARsd58E6qOhEHM1caomqj_r9MOiQ%40mail.gmail.com,
   https://postgr.es/m/CAKJS1f80o98hcfSk8j%3DfdN09S7Sjz%2BvuzhEwbyQqvHJb_sZw0g%40mail.gmail.com

Backpatch-through: 10

doc/src/sgml/monitoring.sgml

@@ -5180,6 +5180,17 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
     </tgroup>
    </table>
 
+  <warning>
+   <para>
+    Using <function>pg_stat_reset()</function> also resets counters that
+    autovacuum uses to determine when to trigger a vacuum or an analyze.
+    Resetting these counters can cause autovacuum to not perform necessary
+    work, which can cause problems such as table bloat or out-dated
+    table statistics.  A database-wide <command>ANALYZE</command> is
+    recommended after the statistics have been reset.
+   </para>
+  </warning>
+
   <para>
    <function>pg_stat_get_activity</function>, the underlying function of
    the <structname>pg_stat_activity</structname> view, returns a set of records