pgsql: Clarify documentation references to pg_stat_get_blocks_fetched

Log Message:
-----------
Clarify documentation references to pg_stat_get_blocks_fetched and
pg_stat_get_blocks_hit, per suggestion from Robert Haas.

Modified Files:
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pgsql/doc/src/sgml:
monitoring.sgml (r1.66 -> r1.67)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.66&r2=1.67)