doc: Add note about generated columns in foreign tables

Explain that it is not enforced that querying a generated column
returns data that is consistent with the data that was stored.  This
is similar to the note about constraints nearby.
Reported-by: Amit Langote <amitlangote09@gmail.com>

doc/src/sgml/ref/create_foreign_table.sgml

@@ -350,6 +350,16 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
     reality.
    </para>
 
+   <para>
+    Similar considerations apply to generated columns.  Stored generated
+    columns are computed on insert or update on the local
+    <productname>PostgreSQL</productname> server and handed to the
+    foreign-data wrapper for writing out to the foreign data store, but it is
+    not enforced that a query of the foreign table returns values for stored
+    generated columns that are consistent with the generation expression.
+    Again, this might result in incorrect query results.
+   </para>
+
    <para>
     While rows can be moved from local partitions to a foreign-table partition
     (provided the foreign data wrapper supports tuple routing), they cannot be