pgsql: doc: Improve descriptions of tup_returned and tup_fetched in pg_

doc: Improve descriptions of tup_returned and tup_fetched in pg_stat_database

Previously the descriptions of tup_returned and tup_fetched columns
in pg_stat_database view were confusing. This commit improves them
so that they represent the following formulas of those columns
more accurately.

* pg_stat_database.tup_returned
    = sum(pg_stat_all_tables.seq_tup_read)
    + sum(pg_stat_all_indexes.idx_tup_read)

* pg_stat_database.tup_fetched
    = sum(pg_stat_all_tables.idx_tup_fetch)

In these formulas, note that the counters for some system catalogs
like pg_database shared across all databases of a cluster are excluded
from the calculations of sum.

Author: Masahiro Ikeda
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/9eeeccdb-5dd7-90f9-2807-a4b5d2b76ca3@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3465c8a8692fb628bc2710fbeedb9aeb534b0400

Modified Files
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doc/src/sgml/monitoring.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)