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