pgsql: Document trace_recovery_messages() parameter as an enum, not

Log Message:
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Document trace_recovery_messages() parameter as an enum, not string.

Fujii Masao

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.255 -> r1.256)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.255&r2=1.256)