pgsql: Add a comment noting that FDWs don't have to implement EXCEPT or

Add a comment noting that FDWs don't have to implement EXCEPT or LIMIT TO.

postgresImportForeignSchema pays attention to IMPORT's EXCEPT and LIMIT TO
options, but only as an efficiency hack, not for correctness' sake.  The
FDW documentation does explain that, but someone using postgres_fdw.c
as a coding guide might not remember it, so let's add a comment here.
Per question from Regina Obe.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5f36096b77fe47015cbac130d1a20d089f202a1e

Modified Files
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contrib/postgres_fdw/postgres_fdw.c |    6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)