Update comments for pg_statistic catalog table

Update comments for pg_statistic catalog table

Make a reminder that pg_stats view needs to be modified whenever a new slot
kind is added.  To prevent situations like 918eee0c49 when pg_stats was
forgotten to be updated.

Also, revise the comment that only non-null, non-empty rows are considered
for the range length histogram.

Discussion: https://postgr.es/m/flat/b67d8b57-9357-7e82-a2e7-f6ce6eaeec67@postgrespro.ru
Author: Egor Rogov, Soumyadeep Chakraborty
Reviewed-by: Tomas Vondra, Justin Pryzby, Jian He