Doc update for pg_stat_statements normalization

Replacing constants in pg_stat_statements is on a best effort basis.
It is not unlikely that on a busy workload with heavy entry deallocation,
the user may observe the query with the constants in pg_stat_statements.

From what I can see, this is because the only time an entry is normalized is
during post_parse_analyze, and the entry may be deallocated by the time query
execution ends. At that point, the original form ( with constants ) of the query
is used.

It is not clear how prevalent this is in real-world workloads, but it's easily reproducible
on a workload with high entry deallocation. Attached are the repro steps on the latest
branch.

I think the only thing to do here is to call this out in docs with a suggestion to increase
pg_stat_statements.max to reduce the likelihood. I also attached the suggested
doc enhancement as well.

Any thoughts?

Regards,

--
Sami Imseih
Amazon Web Services