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