pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e820db5b56b20c68bce2956f8f6c8ba6bfff5735

Modified Files
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doc/src/sgml/monitoring.sgml    | 9 +++++----
src/include/catalog/pg_proc.dat | 6 +++---
2 files changed, 8 insertions(+), 7 deletions(-)