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

Author: MasaoFujii

doc/src/sgml/monitoring.sgml

@@ -3712,7 +3712,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>tup_returned</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of rows returned by queries in this database
+       Number of live rows fetched by sequential scans and index entries returned by index scans in this database
       </para></entry>
      </row>
 
@@ -3721,7 +3721,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>tup_fetched</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of rows fetched by queries in this database
+       Number of live rows fetched by index scans in this database
       </para></entry>
      </row>