doc: Mention AttributeRelationId in FDW validator function description

The documentation has been missing one value in the list of catalog OIDs
that can be given to the validator function of a FDW, as of
AttributeRelationId, when changing the attribute options of a foreign
table.

Author: Ian Lawrence Barwick
Discussion: https://postgr.es/m/CAB8KJ=i16t2yJU_Pq2Z+hnNGWFhagp_bJmzxHZu3ZkOjZm-+rQ@mail.gmail.com
Backpatch-through: 12

doc/src/sgml/fdwhandler.sgml

@@ -67,13 +67,16 @@
      foreign tables using the wrapper.
      The validator function must be registered as taking two arguments, a
      text array containing the options to be validated, and an OID
-     representing the type of object the options are associated with (in
-     the form of the OID of the system catalog the object would be stored
-     in, either
-     <literal>ForeignDataWrapperRelationId</literal>,
-     <literal>ForeignServerRelationId</literal>,
-     <literal>UserMappingRelationId</literal>,
-     or <literal>ForeignTableRelationId</literal>).
+     representing the type of object the options are associated with. The
+     latter corresponds to the OID of the system catalog the object
+     would be stored in, one of:
+     <itemizedlist spacing="compact">
+      <listitem><para><literal>AttributeRelationId</literal></para></listitem>
+      <listitem><para><literal>ForeignDataWrapperRelationId</literal></para></listitem>
+      <listitem><para><literal>ForeignServerRelationId</literal></para></listitem>
+      <listitem><para><literal>ForeignTableRelationId</literal></para></listitem>
+      <listitem><para><literal>UserMappingRelationId</literal></para></listitem>
+     </itemizedlist>
      If no validator function is supplied, options are not checked at object
      creation time or object alteration time.
     </para>