pgsql: docs: Reframe track_io_timing related docs as wait time

docs: Reframe track_io_timing related docs as wait time

With AIO it does not make sense anymore to track the time for each individual
IO, as multiple IOs can be in-flight at the same time. Instead we now track
the time spent *waiting* for IOs.

This should be reflected in the docs. While, so far, we only do a subset of
reads, and no other operations, via AIO, describing the GUC and view columns
as measuring IO waits is accurate for synchronous and asynchronous IO.

Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/5dzyoduxlvfg55oqtjyjehez5uoq6hnwgzor4kkybkfdgkj7ag@rbi4gsmzaczk

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b27f8637ea708c4b6113f0d29a76cb411b430a27

Modified Files
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doc/src/sgml/config.sgml | 4 ++--
doc/src/sgml/monitoring.sgml | 12 ++++++------
2 files changed, 8 insertions(+), 8 deletions(-)