doc: mention unusability of dropped CHECK to verify NOT NULL

It's possible to use a CHECK (col IS NOT NULL) constraint to skip
scanning a table for nulls when adding a NOT NULL constraint on the same
column.  However, if the CHECK constraint is dropped on the same command
that the NOT NULL is added, this fails, i.e., makes the NOT NULL addition
slow.  The best we can do about it at this stage is to document this so
that users aren't taken by surprise.

(In Postgres 18 you can directly add the NOT NULL constraint as NOT
VALID instead, so there's no longer much use for the CHECK constraint,
therefore no point in building mechanism to support the case better.)

Reported-by: Andrew <psy2000usa@yahoo.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/175385113607.786.16774570234342968908@wrigleys.postgresql.org

doc/src/sgml/ref/alter_table.sgml

@@ -237,9 +237,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       provided none of the records in the table contain a
       <literal>NULL</literal> value for the column.  Ordinarily this is
       checked during the <literal>ALTER TABLE</literal> by scanning the
-      entire table; however, if a valid <literal>CHECK</literal> constraint is
-      found which proves no <literal>NULL</literal> can exist, then the
-      table scan is skipped.
+      entire table;
+      however, if a valid <literal>CHECK</literal> constraint exists
+      (and is not dropped in the same command) which proves no
+      <literal>NULL</literal> can exist, then the table scan is skipped.
      </para>
 
      <para>