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

doc/src/sgml/ref/explain.sgml

@@ -171,8 +171,9 @@ ROLLBACK;
      <para>
       Include information on buffer usage. Specifically, include the number of
       shared blocks hit, read, dirtied, and written, the number of local blocks
-      hit, read, dirtied, and written, and the number of temp blocks read and
-      written.
+      hit, read, dirtied, and written, the number of temp blocks read and
+      written, and the time spent reading and writing data file blocks
+      (in milliseconds) if <xref linkend="guc-track-io-timing"/> is enabled.
       A <emphasis>hit</emphasis> means that a read was avoided because the block was
       found already in cache when needed.
       Shared blocks contain data from regular tables and indexes;