docs: Enhance the pg_stat_checkpointer view documentation.

docs: Enhance the pg_stat_checkpointer view documentation.

This commit updates the documentation for the pg_stat_checkpointer view
to clarify what kind of checkpoints or restartpoints each counter tracks.
This makes it easier to understand the meaning of each counter.

Previously, the num_requested description included "backend,"
which could be misleading since requests come from other sources as well.
This commit also removes "backend" from the description of num_requested,
to avoid confusion.

Author: Fujii Masao
Reviewed-by: Anton A. Melnikov
Discussion: https://postgr.es/m/4640258e-d959-4cf0-903c-cd02389c3e05@oss.nttdata.com