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/[email protected]

Branch
------
master

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

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

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