Add FDW documentation notes about insert and update tuple routing and COPY.

Author: Laurenz Albe and Etsuro Fujita
Reviewed-by: Laurenz Albe and Amit Langote
Backpatch-through: 11 where support for that by FDWs was added
Discussion: https://postgr.es/m/bf36a0288e8f31b4f2f40952e225bf892dc1ffc5.camel@cybertec.at

doc/src/sgml/fdwhandler.sgml

@@ -593,6 +593,14 @@ ExecForeignInsert(EState *estate,
      with an error message.
     </para>
 
+    <para>
+     Note that this function is also called when inserting routed tuples into
+     a foreign-table partition or executing <command>COPY FROM</command> on
+     a foreign table, in which case it is called in a different way than it
+     is in the <command>INSERT</command> case.  See the callback functions
+     described below that allow the FDW to support that.
+    </para>
+
     <para>
 <programlisting>
 TupleTableSlot *
@@ -751,6 +759,13 @@ BeginForeignInsert(ModifyTableState *mtstate,
      <literal>NULL</literal>, no action is taken for the initialization.
     </para>
 
+    <para>
+     Note that if the FDW does not support routable foreign-table partitions
+     and/or executing <command>COPY FROM</command> on foreign tables, this
+     function or <function>ExecForeignInsert</function> subsequently called
+     must throw error as needed.
+    </para>
+
     <para>
 <programlisting>
 void