pgsql: Add comment that pg_dump 'append' format is used only by pg_dump,

Log Message:
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Add comment that pg_dump 'append' format is used only by pg_dump, per
Dave Page.

Modified Files:
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pgsql/src/bin/pg_dump:
pg_dump.c (r1.463 -> r1.464)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.463&r2=1.464)