Adjust some comments about structure properties in pg_stat.h
One comment of PgStat_TableCounts mentioned that its pending stats use
memcmp() to check for the all-zero case if there is any activity. This
is not true since
07e9e28b56, as pg_memory_is_all_zeros() is used.
PgStat_FunctionCounts incorrectly documented that it relied on memcpy().
This has never been correct, and not relevant because function
statistics do not have an all-zero check for pending stats.
Checkpoint and bgwriter statistics have been always relying on memcmp()
or pg_memory_is_all_zeros() (since
07e9e28b56 for the latter), and never
mentioned the dependency on event counters for their all-zero checks.
Let's document these properties, like the table statistics.
Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Z1hNLvcPgVLPxCoc@ip-10-97-1-34.eu-west-3.compute.internal