doc: Update track_io_timing documentation to mention pg_stat_io.

doc: Update track_io_timing documentation to mention pg_stat_io.

The I/O timing information collected when track_io_timing is
enabled is now documented to appear in the pg_stat_io view,
which was previously not mentioned.

This commit also enhances the description of track_io_timing
to clarify that it monitors not only block read and write
but also block extend and fsync operations. Additionally,
the description of track_wal_io_timing has been improved
to mention both WAL write and WAL fsync monitoring.

Backpatch to v16 where pg_stat_io was added.

Author: Hajime Matsunaga
Reviewed-by: Melanie Plageman, Nazir Bilal Yavuz, Fujii Masao
Discussion: https://postgr.es/m/TYWPR01MB10742EE4A6F34C33061429D38A4D52@TYWPR01MB10742.jpnprd01.prod.outlook.com