pgsql: 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(at)postgrespro(dot)ru
Author: Egor Rogov, Soumyadeep Chakraborty
Reviewed-by: Tomas Vondra, Justin Pryzby, Jian He

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/441c8a31349105590e778fe7dd1db0d0c89299df

Modified Files
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src/include/catalog/pg_statistic.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)