Improve documentation about foreign data wrapper validator functions.

Modified version of a patch by Shigeru Hanada.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/52a4255055f2f50c7f20917a47e2da04b8068fd6

Modified Files
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doc/src/sgml/fdwhandler.sgml |   16 +++++++++++-----
1 files changed, 11 insertions(+), 5 deletions(-)

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