pgsql: Add docs about postgres_fdw's setting of search_path and other G

Add docs about postgres_fdw's setting of search_path and other GUCs.

This behavior wasn't documented, but it should be because it's user-visible
in triggers and other functions executed on the remote server.
Per question from Adam Fuchs.

Back-patch to 9.3 where postgres_fdw was added.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/9a18a2bfb9f93e4a1aa405e752608746e04619f2

Modified Files
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doc/src/sgml/postgres-fdw.sgml |   35 +++++++++++++++++++++++++++++++++++
1 file changed, 35 insertions(+)