FOREIGN TABLE doc fix

Hi hackers,

At first I've posted to pgsql-docs but there is no reply at present.

    http://archives.postgresql.org/pgsql-docs/2011-06/msg00006.php

So I post revised patch to pgsql-hackers, because the patch is for
documents under development for 9.1 beta2. Please let me know if this
was wrong list to discuss these issues.

Attached patch includes fixes for FOREIGN TABLE documents:

1) "NOT NULL" should be added to syntax of ALTER FOREIGN TABLE ADD
COLUMN command.

2) Mentions about unsupported features should be removed from ALTER
FOREIGN TABLE document. They had been proposed but haven't been
committed for 9.1. These unsupported features are not mentioned in
CREATE FOREIGN TABLE document.

  - table inheritance
  - oid system column
  - CHECK constraint
  - per-column FDW options

3) It would be useful to mention differences between ordinary tables and
foreign tables in CREATE FOREIGN TABLE document.

  - NOT NULL constraint is just for optimization, at least at present
  - serial/bigserial can't be used because they need default value

The description in the patch are based on the advice which were posted
by Thom Brown and Robert Haas in the thread:

    http://archives.postgresql.org/pgsql-hackers/2011-03/msg01949.php

Regards,
--
Shigeru Hanada