pgsql: Adjust some comments about structure properties in pg_stat.h

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4766438aa317e85f3d762847b2b009f91f530b6f

Modified Files
--------------
src/include/pgstat.h | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)