doc: Document range_intersect_agg(anymultirange)

It already existed but was not mentioned in the documentation.  (Only
the anyrange variant was listed.)

Author: Paul Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Chapman Flack <chap@anastigmatix.net>
Discussion: https://www.postgresql.org/message-id/flat/007ef255-35ef-fd26-679c-f97e7a7f30c2@illuminatedcomputing.com

Author: petere

doc/src/sgml/func.sgml

@@ -20022,6 +20022,11 @@ SELECT NULLIF(value, '(none)') ...
          <type>anyrange</type> )
         <returnvalue>anymultirange</returnvalue>
        </para>
+       <para role="func_signature">
+        <function>range_intersect_agg</function> ( <parameter>value</parameter>
+         <type>anymultirange</type> )
+        <returnvalue>anymultirange</returnvalue>
+       </para>
        <para>
         Computes the intersection of the non-null input values.
        </para></entry>