pgsql: Document that Python 2.3 requires cdecimal module for full funct

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.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/a8ee81822e43120e1b31949b07af1adadcbeffc1

Modified Files
--------------
doc/src/sgml/installation.sgml | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

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