pgsql: Improve the manual's discussion of partitioning.

Log Message:
-----------
Improve the manual's discussion of partitioning.  Recommend using a
trigger instead of a rule to redirect insertions, use NEW.* notation
where appropriate, some other updates and adjustments.  David Fetter
and Tom Lane

Modified Files:
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    pgsql/doc/src/sgml:
        ddl.sgml (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml?r1=1.77&r2=1.78)