Improve FDW document (for 9.1)

Hi,

During reading Albe's patch for FDW validator attached to "Bug in
SQL/MED?" posted to pgsql-hackers[1], I noticed that neither ALTER
commands nor USER MAPPING is mentioned in the description of FDW
validator in "Writing A Foreign Data Wrapper"[2]. The validator
function is also called for ALTER commands and USER MAPPING
manipulation, if any FDW option remains.

FYI, Albe's patch is for calling validator even if no option remains.

[1]http://archives.postgresql.org/pgsql-hackers/2011-06/msg02230.php
[2]http://developer.postgresql.org/pgdocs/postgres/fdwhandler.html

Please find attached a patch which adds a mention of ALTER and USER MAPPING.

Regards,
--
Shigeru Hanada