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@leadboat.com>
Discussion: https://postgr.es/m/5dzyoduxlvfg55oqtjyjehez5uoq6hnwgzor4kkybkfdgkj7ag@rbi4gsmzaczk

doc/src/sgml/config.sgml

@@ -8568,7 +8568,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </term>
       <listitem>
        <para>
-        Enables timing of database I/O calls.  This parameter is off by
+        Enables timing of database I/O waits.  This parameter is off by
         default, as it will repeatedly query the operating system for
         the current time, which may cause significant overhead on some
         platforms.  You can use the <xref linkend="pgtesttiming"/> tool to
@@ -8602,7 +8602,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </term>
       <listitem>
        <para>
-        Enables timing of WAL I/O calls. This parameter is off by default,
+        Enables timing of WAL I/O waits. This parameter is off by default,
         as it will repeatedly query the operating system for the current time,
         which may cause significant overhead on some platforms.
         You can use the <application>pg_test_timing</application> tool to

doc/src/sgml/monitoring.sgml

@@ -2747,7 +2747,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         <structfield>read_time</structfield> <type>double precision</type>
        </para>
        <para>
-        Time spent in read operations in milliseconds (if
+        Time spent waiting for read operations in milliseconds (if
         <xref linkend="guc-track-io-timing"/> is enabled and
         <varname>object</varname> is not <literal>wal</literal>,
         or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@@ -2785,7 +2785,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         <structfield>write_time</structfield> <type>double precision</type>
        </para>
        <para>
-        Time spent in write operations in milliseconds (if
+        Time spent waiting for write operations in milliseconds (if
         <xref linkend="guc-track-io-timing"/> is enabled and
         <varname>object</varname> is not <literal>wal</literal>,
         or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@@ -2813,7 +2813,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         <structfield>writeback_time</structfield> <type>double precision</type>
        </para>
        <para>
-        Time spent in writeback operations in milliseconds (if
+        Time spent waiting for writeback operations in milliseconds (if
         <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero). This
         includes the time spent queueing write-out requests and, potentially,
         the time spent to write out the dirty data.
@@ -2849,7 +2849,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         <structfield>extend_time</structfield> <type>double precision</type>
        </para>
        <para>
-        Time spent in extend operations in milliseconds. (if
+        Time spent waiting for extend operations in milliseconds. (if
         <xref linkend="guc-track-io-timing"/> is enabled and
         <varname>object</varname> is not <literal>wal</literal>,
         or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@@ -2923,7 +2923,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
         <structfield>fsync_time</structfield> <type>double precision</type>
        </para>
        <para>
-        Time spent in fsync operations in milliseconds (if
+        Time spent waiting for fsync operations in milliseconds (if
         <xref linkend="guc-track-io-timing"/> is enabled and
         <varname>object</varname> is not <literal>wal</literal>,
         or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@@ -3010,7 +3010,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
 
   <note>
    <para>
-    Columns tracking I/O time will only be non-zero when
+    Columns tracking I/O wait time will only be non-zero when
     <xref linkend="guc-track-io-timing"/> is enabled. The user should be
     careful when referencing these columns in combination with their
     corresponding I/O operations in case <varname>track_io_timing</varname>