Add explanations which are influenced by track_io_timing

When track_io_timing is on, I/O timing information is
displayed in pg_stat_database, in the output of EXPLAIN
when the BUFFERS option is used, and in pg_stat_statements
as documented in [1].

This is also described in the manual for pg_stat_statements
[2], however, manuals for pg_stat_database and EXPLAIN
doesn't refer to it.

I think it'll be better to add descriptions to both of them