pgsql: Do a round of copy-editing for the system catalog chapter.

Log Message:
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Do a round of copy-editing for the system catalog chapter. In particular,
consistently capitalize the content of the "Description" column but do
not include a terminating period, as is the convention elsewhere in the
docs. Also, remove the "References" column from catalog that do not
have any referencing columns, for the sake of brevity. Make various
other SGML and grammar fixes.

Modified Files:
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    pgsql/doc/src/sgml:
        catalogs.sgml (r2.136 -> r2.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.136&r2=2.137)