Document why you can't use date_trunc("week") on intervals.

Author: bmomjian

doc/src/sgml/func.sgml

@@ -7050,6 +7050,11 @@ date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>
     </simplelist>
    </para>
 
+   <para>
+    <literal>week</literal> is not supported for <type>interval</>
+    values because months usually have fractional weeks.
+   </para>
+
    <para>
     Examples:
 <screen>