Doc: update libpq's documentation for PQfn().

Mention specifically that you can't call aggregates, window functions,
or procedures this way (the inability to call SRFs was already
mentioned).

Also, the claim that PQfn doesn't support NULL arguments or results
has been a lie since we invented protocol 3.0.  Not sure why this
text was never updated for that, but do it now.

Discussion: https://postgr.es/m/2039442.1615317309@sss.pgh.pa.us

doc/src/sgml/libpq.sgml

@@ -5760,15 +5760,32 @@ typedef struct
 
   <para>
    <function>PQfn</function> always returns a valid
-   <structname>PGresult</structname> pointer. The result status should be
+   <structname>PGresult</structname> pointer, with
+   status <literal>PGRES_COMMAND_OK</literal> for success
+   or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
+   The result status should be
    checked before the result is used.   The caller is responsible for
    freeing  the  <structname>PGresult</structname>  with
    <xref linkend="libpq-PQclear"/> when it is no longer needed.
   </para>
 
   <para>
-   Note that it is not possible to handle null arguments, null results,
-   nor set-valued results when using this interface.
+   To pass a NULL argument to the function, set
+   the <parameter>len</parameter> field of that parameter structure
+   to <literal>-1</literal>; the <parameter>isint</parameter>
+   and <parameter>u</parameter> fields are then irrelevant.
+  </para>
+
+  <para>
+   If the function returns NULL, <parameter>*result_len</parameter> is set
+   to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not
+   modified.
+  </para>
+
+  <para>
+   Note that it is not possible to handle set-valued results when using
+   this interface.  Also, the function must be a plain function, not an
+   aggregate, window function, or procedure.
   </para>
 
  </sect1>