pgsql: docs: Improve the description of num_timed column in pg_stat_che

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a7c39db5eb342dd491dacf39546486c6a539ea43

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)