Fix #54863 by clarifying return values

Closes GH-318.

Author: afilina

reference/pgsql/functions/pg-fetch-array.xml

@@ -89,6 +89,7 @@
   <para>
    &false; is returned if <parameter>row</parameter> exceeds the number
    of rows in the set, there are no more rows, or on any other error.
+   Fetching from the result of a query other than SELECT will also return &false;.
   </para>
  </refsect1>