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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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/69892d58c9881acac934629187c7301f8e296eb4

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