doc: Mention cost-based delays for total_[auto]{vacuum,analyze}_time

30a6ed0ce4 has added four attributes to pg_stat_all_tables to track the
cumulative time spent in [auto]vacuum and [auto]analyze.  It was not
mentioned that the vacuum cost-based delays are included in these
numbers, which could be confusing now that the delays are included in
the vacuum progress view (bb8dff9995).

This commit adds an extra note about this matter.

Reported-by: Magnus Hagander <magnus@hagander.net>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/CABUevEz9v1ZNToPyD98JnWDGZgG=SmPZKkSNzU9hXQ-nGTQF0g@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -4066,7 +4066,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        <structfield>total_vacuum_time</structfield> <type>double precision</type>
       </para>
       <para>
-       Total time this table has been manually vacuumed, in milliseconds
+       Total time this table has been manually vacuumed, in milliseconds.
+       (This includes the time spent sleeping due to cost-based delays.)
       </para></entry>
      </row>
 
@@ -4076,7 +4077,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Total time this table has been vacuumed by the autovacuum daemon,
-       in milliseconds
+       in milliseconds. (This includes the time spent sleeping due to
+       cost-based delays.)
       </para></entry>
      </row>
 
@@ -4085,7 +4087,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        <structfield>total_analyze_time</structfield> <type>double precision</type>
       </para>
       <para>
-       Total time this table has been manually analyzed, in milliseconds
+       Total time this table has been manually analyzed, in milliseconds.
+       (This includes the time spent sleeping due to cost-based delays.)
       </para></entry>
      </row>
 
@@ -4095,7 +4098,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Total time this table has been analyzed by the autovacuum daemon,
-       in milliseconds
+       in milliseconds. (This includes the time spent sleeping due to
+       cost-based delays.)
       </para></entry>
      </row>
     </tbody>