Clarify that pg_stat_activity.query has a length limit.

There was always documentation of the GUC that controlled what the
limit actually was, but previously the documentation of the field
itself made no mention of that limit.

Ian Barwick

doc/src/sgml/monitoring.sgml

@@ -785,7 +785,9 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
      <entry>Text of this backend's most recent query. If
       <structfield>state</> is <literal>active</> this field shows the
       currently executing query. In all other states, it shows the last query
-      that was executed.
+      that was executed. By default the query text is truncated at 1024
+      characters; this value can be changed via the parameter
+      <xref linkend="guc-track-activity-query-size">.
      </entry>
     </row>
    </tbody>