Document that Python 2.3 requires cdecimal module for full functionality.

This has been true for some time, but we were leaving users to discover it
the hard way.

Back-patch to 9.2.  It might've been true before that, but we were claiming
Python 2.2 compatibility before that, so I won't guess at the exact
requirements back then.
Silence compiler warnings in new jsonb code.

Amit Kapila.
contrib/test_decoding:  adjust expected output

Expected output has changed because of psql replica identity output
changes.

Reported by Christoph Berg
Improve documentation note about Python 2.3 and cdecimal.

Explain exactly what fails (ie, function arguments of type numeric)
if you don't have it.