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

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl