Improve documentation for pg_stat_checkpointer fields

Improve documentation for pg_stat_checkpointer fields

pg_stat_checkpointer contains statistics for checkpoints and restartpoints.
Before 12915a58eec9 documentation said only about checkpoints implying that
restartpoint is the variation of checkpoint.  12915a58eec9 introduced
new separate statistics fields for restartpoints.  This commit explicitly
documents fields that are relevant for both checkpoints and restartpoints.

Reported-by: Magnus Hagander
Discussion: https://postgr.es/m/CABUevExav5-SR0x%2BG9kBUMV0G8XsvSUfuyyqmYBBJi6VHns6sw%40mail.gmail.com
Reviewed-by: Anton A. Melnikov