Document asynchronous CHECK constraint validation

Fixes #4362.

Summary of changes:

- Update `CHECK` constraint page to note that validation is now
  asynchronous.

- Update 'Add Constraint' page to note that validation is now
  asynchronous, and to give an example of adding a CHECK constraint to a
  column added earlier in the same transaction.

v19.1/add-constraint.md

@@ -55,9 +55,33 @@ Adding the [`CHECK` constraint](check.html) requires that all of a column's valu
 
 {% include copy-clipboard.html %}
 ~~~ sql
-> ALTER TABLE orders ADD CONSTRAINT total_0_check CHECK (total > 0);
+> ALTER TABLE orders ADD CONSTRAINT check_id_non_zero CHECK (id > 0);
 ~~~
 
+<span class="version-tag">New in v19.1</span>: Check constraints can be added to columns that were created earlier in the transaction.  For example:
+
+{% include copy-clipboard.html %}
+~~~ sql
+> BEGIN;
+> ALTER TABLE customers ADD COLUMN gdpr_status STRING;
+> ALTER TABLE customers ADD CONSTRAINT check_gdpr_status CHECK (gdpr_status IN ('yes', 'no', 'unknown'));
+> COMMIT;
+~~~
+
+~~~
+BEGIN
+ALTER TABLE
+ALTER TABLE
+COMMIT
+~~~
+
+{{site.data.alerts.callout_info}}
+The entire transaction will be rolled back, including any new columns that were added, in the following cases:
+
+- If an existing column is found containing values that violate the new constraint.
+- If a new column has a default value or is a [computed column](computed-columns.html) that would have contained values that violate the new constraint.
+{{site.data.alerts.end}}
+
 ### Add the foreign key constraint with `CASCADE`
 
 To add a foreign key constraint, use the steps shown below.

v19.1/check.md

@@ -6,10 +6,10 @@ toc: true
 
 The `CHECK` [constraint](constraints.html) specifies that values for the column in [`INSERT`](insert.html) or [`UPDATE`](update.html) statements must return `TRUE` or `NULL` for a Boolean expression. If any values return `FALSE`, the entire statement is rejected.
 
-
 ## Details
 
-- If you add a `CHECK` constraint to an existing table, added values, along with any updates to current values, are checked. To check the existing rows, use [`VALIDATE CONSTRAINT`](validate-constraint.html).
+- <span class="version-tag">New in v19.1</span>: If you add a `CHECK` constraint to an existing table, CockroachDB will run a background job to validate existing table data in the process of adding the constraint. If a row is found that violates the constraint during the validation step, the [`ADD CONSTRAINT`](add-constraint.html) statement will fail. This differs from previous versions of CockroachDB, which allowed you to add a check constraint that was enforced for writes but could be violated by rows that existed prior to adding the constraint.
+- <span class="version-tag">New in v19.1</span>: Check constraints can be added to columns that were created earlier in the same transaction.  For an example, see [Add the `CHECK` constraint](add-constraint.html#add-the-check-constraint).
 - `CHECK` constraints may be specified at the column or table level and can reference other columns within the table. Internally, all column-level `CHECK` constraints are converted to table-level constraints so they can be handled consistently.
 - You can have multiple `CHECK` constraints on a single column but ideally, for performance optimization, these should be combined using the logical operators. For example: