pgsql: Rename Online Backup to Continuous Archiving.

Log Message:
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Rename Online Backup to Continuous Archiving.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.77 -> r2.78)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.77&r2=2.78)
config.sgml (r1.47 -> r1.48)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.47&r2=1.48)
func.sgml (r1.307 -> r1.308)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.307&r2=1.308)
wal.sgml (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml.diff?r1=1.38&r2=1.39)