doc: Improve descriptions of tup_returned and tup_fetched in pg_stat_database

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