pgsql: Document cross-version compatibility issues for contrib/postgres

Document cross-version compatibility issues for contrib/postgres_fdw.

One of the use-cases for postgres_fdw is extracting data from older PG
servers, so cross-version compatibility is important. Document what we
can do here, and further annotate some of the coding choices that create
compatibility constraints. In passing, remove one unnecessary
incompatibility with old servers, namely assuming that we didn't need to
quote the timezone name 'UTC'.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5b86fedfb57ea943f883a13c6d50c5a9e2a1cc57

Modified Files
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contrib/postgres_fdw/connection.c | 15 +++++++++++----
contrib/postgres_fdw/postgres_fdw.c | 3 +++
doc/src/sgml/postgres-fdw.sgml | 20 ++++++++++++++++++++
3 files changed, 34 insertions(+), 4 deletions(-)