pgsql: Convert explanation of interval change into something resembling

Log Message:
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Convert explanation of interval change into something resembling grammatical
English, and fix the incorrect example.

Modified Files:
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pgsql/doc/src/sgml:
release.sgml (r1.392 -> r1.393)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml.diff?r1=1.392&r2=1.393)