pgsql: Briefly document in two places that pg_dump and pg_dumpall cannot

Log Message:
-----------
Briefly document in two places that pg_dump and pg_dumpall cannot be
used for continuous archiving.

Modified Files:
--------------
pgsql/doc/src/sgml:
backup.sgml (r2.143 -> r2.144)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.143&r2=2.144)