Document auto-validation of foreign keys

Fixes #4891.

v19.2/add-constraint.md

@@ -32,7 +32,7 @@ The user must have the `CREATE` [privilege](authorization.html#assign-privileges
 -----------|-------------
  `table_name` | The name of the table containing the column you want to constrain.
  `constraint_name` | The name of the constraint, which must be unique to its table and follow these [identifier rules](keywords-and-identifiers.html#identifiers).
- `constraint_elem` | The [`CHECK`](check.html), [foreign key](foreign-key.html), [`UNIQUE`](unique.html) constraint you want to add. <br/><br/>Adding/changing a `DEFAULT` constraint is done through [`ALTER COLUMN`](alter-column.html). <br/><br/>Adding/changing the table's `PRIMARY KEY` is not supported through `ALTER TABLE`; it can only be specified during [table creation](create-table.html).
+ `constraint_elem` | The [`CHECK`](check.html), [foreign key](foreign-key.html), [`UNIQUE`](unique.html) constraint you want to add.<br/><br/>Adding/changing a `DEFAULT` constraint is done through [`ALTER COLUMN`](alter-column.html).<br/><br/>Adding/changing the table's `PRIMARY KEY` is not supported through `ALTER TABLE`; it can only be specified during [table creation](create-table.html).<br><br><span class="version-tag">New in v19.2:</span> Applying a foreign key constraint now validates existing rows in addition to enforcing conformance for new rows. As such, it is no longer necessary to use [`VALIDATE CONSTRAINT`](validate-constraint.html) for foreign keys.
 
 ## Viewing schema changes
 

v19.2/validate-constraint.md

@@ -6,6 +6,8 @@ toc: true
 
 The `VALIDATE CONSTRAINT` [statement](sql-statements.html) is part of `ALTER TABLE` and checks whether values in a column match a [constraint](constraints.html) on the column. This statement is especially useful after applying a constraint to an existing column via [`ADD CONSTRAINT`](add-constraint.html). In this case, `VALIDATE CONSTRAINT` can be used to find values already in the column that do not match the constraint.
 
+<span class="version-tag">New in v19.2:</span> Applying a foreign key constraint via `ADD CONSTRAINT` now validates existing rows in addition to enforcing conformance for new rows. As such, it is no longer necessary to use [`VALIDATE CONSTRAINT`](validate-constraint.html) for foreign keys.
+
 {% include {{ page.version.version }}/sql/combine-alter-table-commands.md %}
 
 ## Required privileges
@@ -29,20 +31,20 @@ The user must have the `CREATE` [privilege](authorization.html#assign-privileges
 
 {% include {{ page.version.version }}/misc/schema-change-view-job.md %}
 
-## Examples
+## Example
 
-In [`ADD CONSTRAINT`](add-constraint.html), we [added a foreign key constraint](add-constraint.html#add-the-foreign-key-constraint-with-cascade) like so:
+In [`ADD CONSTRAINT`](add-constraint.html), we [added a `CHECK` constraint](add-constraint.html#add-the-check-constraint) like so:
 
 {% include copy-clipboard.html %}
 ~~~ sql
-> ALTER TABLE orders ADD CONSTRAINT customer_fk FOREIGN KEY (customer_id) REFERENCES customers (id) ON DELETE CASCADE;
+> ALTER TABLE orders ADD CONSTRAINT check_id_non_zero CHECK (id > 0);
 ~~~
 
-In order to ensure that the data added to the `orders` table prior to the creation of the `customer_fk` constraint conforms to that constraint, run the following:
+In order to ensure that the data added to the `orders` table prior to the creation of the `check_id_non_zer` constraint conforms to that constraint, run the following:
 
 {% include copy-clipboard.html %}
 ~~~ sql
-> ALTER TABLE orders VALIDATE CONSTRAINT customer_fk;
+> ALTER TABLE orders VALIDATE CONSTRAINT check_id_non_zero;
 ~~~
 
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