Fix comments related to pending statistics

Hi hackers,

while working on [1], I came across 2 comments in pgstat.h that I think are 
not correct.

1. One linked to PgStat_TableCounts pending stats, mentioning the use of
memcmp() against zeroes to detect whether there are any stats updates to apply.

This is not true anymore as of 07e9e28b56.

2. One linked to PgStat_FunctionCounts pending stats, mentioning the use of
memcmp() against zeroes to detect whether there are any pending stats: I think
it has never been the case.

Please find attached a patch to fix those comments.

[1]: https://www.postgresql.org/message-id/flat/ZlGYokUIlERemvpB%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com