Document atthasmissing default optimization avoids verification table scan

When PG11 added the ability for ALTER TABLE ADD COLUMN to set a constant
default value without rewriting the table the doc changes did not note
how the new feature interplayed with ADD COLUMN DEFAULT NOT NULL.
Previously such a new column required a verification table scan to
ensure no values were null. That scan happens under an exclusive lock on
the table, so it can have a meaningful impact on database "accessible
uptime".

I've attached a patch to document that the new mechanism also
precludes that scan.

Thanks,
James Coleman