Improve descriptions of some pg_stat_checkpoints functions in pg_proc.dat.

Improve descriptions of some pg_stat_checkpoints functions in pg_proc.dat.

Previously, the descriptions of pg_stat_get_checkpointer_num_requested(),
pg_stat_get_checkpointer_restartpoints_requested(),
and pg_stat_get_checkpointer_restartpoints_performed() in pg_proc.dat
referred to "backend". This was misleading because these functions report
the number of checkpoints or restartpoints requested or performed
by other than backends as well.

This commit removes "backend" from these descriptions to avoid confusion.

Bump catalog version.

Idea from Anton A. Melnikov
Author: Fujii Masao
Reviewed-by: Anton A. Melnikov
Discussion: https://postgr.es/m/8e5f353f-8b31-4a8e-9cfa-c037f22b4aee@postgrespro.ru