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
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2a33b796ee27732c1854f41594329afbb9ffba3d

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