File_fdw documentation patch to clarify OPTIONS clause

The file_fdw documentation for the OPTIONS clause references the COPY command's documentation.  In the case of COPY, the value for some options (e.g. HEADER, OIDS, ...) is optional.  While the file_fdw documentation makes no mention of it, the values for all options are required.

Attached is a patch to add a note to the docs mentioning this fact.