Clarify known limitation on schema changes within transactions

[jseldess]

This known limitation states that arranging DML statements so they come after DDL statements should work, but it contradicts a few of these examples. And @joshimhoff confirmed that you can't, for example, add a column and then update its value within a single transaction.

[rafiss]

The docs on the known limitations page should also be updated. The same wording is used in this section of the page linked in the original issue.

cockroachdb/cockroach#26508 tracks the mized DDL/DML transactions issue.

Here are some super simple cases that don't work:

create table t (a int primary key);
begin;
savepoint cockroach_restart;
alter table t add column e int;
select a,e from t;

ERROR: column "e" does not exist
SQLSTATE: 42703

create table t (a int primary key);
begin;
savepoint cockroach_restart;
alter table t add column e int;
update t set e = 1;

ERROR: column "e" does not exist
SQLSTATE: 42703

Note that the example in this section of the docs DOES work. (I.e., if the table is created inside the transaction, it works.)

  BEGIN;
  SAVEPOINT cockroach_restart;
  CREATE TABLE fruits (
        id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
        name STRING,
        color STRING
    );
  INSERT INTO fruits (name, color) VALUES ('apple', 'red');
  ALTER TABLE fruits ADD COLUMN inventory_count INTEGER DEFAULT 5;
  ALTER TABLE fruits ADD CONSTRAINT name CHECK (name IN ('apple', 'banana', 'orange'));
  SELECT name, color, inventory_count FROM fruits;
  RELEASE SAVEPOINT cockroach_restart;
  COMMIT;