pgsql: doc: Add note about I/O timing information in EXPLAIN and pg_sta

doc: Add note about I/O timing information in EXPLAIN and pg_stat_database.

Explain that the followings are tracked only when track_io_timing GUC
is enabled.

- blk_read_time and blk_write_time in pg_stat_database
- time spent reading and writing data file blocks in EXPLAIN output
   with BUFFERS option

Whther track_io_timing is enabled affects also blk_read_time and
blk_write_time in pg_stat_statements, but which was already documented.

Author: Atsushi Torikoshi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CACZ0uYHo_NwbxpLH76OGF-O=13tkR0ZM0zeyGEhZ+JEXZVRyCA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eaae947e2b99a84c8f321fe084d87daff0f77d02

Modified Files
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doc/src/sgml/monitoring.sgml  | 6 ++++--
doc/src/sgml/ref/explain.sgml | 5 +++--
2 files changed, 7 insertions(+), 4 deletions(-)