docs: Improve the description of num_timed column in pg_stat_checkpointer.

The previous documentation stated that num_timed reflects the number of
scheduled checkpoints performed. However, checkpoints may be skipped
if the server has been idle, and num_timed counts both skipped and completed
checkpoints. This commit clarifies the description to make it clear that
the counter includes both skipped and completed checkpoints.

Back-patch to v17 where pg_stat_checkpointer was added.

Author: Fujii Masao
Reviewed-by: Alexander Korotkov
Discussion: https://postgr.es/m/9ea77f40-818d-4841-9dee-158ac8f6e690@oss.nttdata.com

Author: MasaoFujii

doc/src/sgml/monitoring.sgml

@@ -3051,7 +3051,10 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        <structfield>num_timed</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of scheduled checkpoints that have been performed
+       Number of scheduled checkpoints due to timeout.
+       Note that checkpoints may be skipped if the server has been idle
+       since the last one, and this value counts both completed and
+       skipped checkpoints
       </para></entry>
      </row>