Avoid row-processing-order dependency in postgres_fdw regression test.

A test intended to provoke an error on the remote side was coded in such
a way that multiple rows should be updated, so the output would vary
depending on which one was processed first.  Per buildfarm.
Fix orthotypographical mistake

Apparently I lost some of the edits I had done on this page for commit
0ac5ad5134.

Per note from Etsuro Fujita, although I didn't use his patch.

Make some of the wording in the affected section a bit more complete,
too.
Allow default expressions to be attached to columns of foreign tables.

There's still some discussion about exactly how postgres_fdw ought to
handle this case, but there seems no debate that we want to allow defaults
to be used for inserts into foreign tables.  So remove the core-code
restrictions that prevented it.

While at it, get rid of the special grammar productions for CREATE FOREIGN
TABLE, and instead add explicit FEATURE_NOT_SUPPORTED error checks for the
disallowed cases.  This makes the grammar a shade smaller, and more
importantly results in much more intelligible error messages for
unsupported cases.  It's also one less thing to fix if we ever start
supporting constraints on foreign tables.
Fix contrib/postgres_fdw's handling of column defaults.

Adopt the position that only locally-defined defaults matter.  Any defaults
defined in the remote database do not affect insertions performed through
a foreign table (unless they are for columns not known to the foreign
table).  While it'd arguably be more useful to permit remote defaults to be
used, making that work in a consistent fashion requires far more work than
seems possible for 9.3.
Fix documentation oversight.

Mention that PlanForeignModify's result must be copiable by copyObject.