doc: Clarify that file_fdw options require values.

Mike Blackwell and Robert Haas

doc/src/sgml/file-fdw.sgml

@@ -111,6 +111,14 @@
 
  </variablelist>
 
+ <para>
+  Note that while <command>COPY</> allows options such as OIDS and HEADER 
+  to be specified without a corresponding value, the foreign data wrapper
+  syntax requires a value to be present in all cases.  To activate 
+  <command>COPY</> options normally supplied without a value, you can
+  instead pass the value TRUE. 
+ </para>
+
  <para>
   A column of a foreign table created using this wrapper can have the
   following options: