docs: re-add paragraph about ON DELETE CASCADE (#2513)

this is an important distinction. Users open issues because they dont
understand the feature scope, e.g. cap/issues/issues/20464

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Co-authored-by: Steffen Weinstock <79531202+stewsk@users.noreply.github.com>
Co-authored-by: René Jeglinsky <rene.jeglinsky@sap.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Mahati Shankar <93712176+smahati@users.noreply.github.com>

Author: 5 people

guides/databases/cdl-to-ddl.md

@@ -611,7 +611,40 @@ CREATE TABLE Books ( ...
 :::
 
 > [!tip] Consider using <code>@assert.target</code> instead
-> Database constraints are meant to protect against data corruption due to programming errors. Prefer using the [`@assert.target`](../services/constraints#assert-target) for application-level input validation, which is more tuned for typical application scenarios, with error messages taylored for end users.
+> Database constraints are meant to protect against data corruption due to programming errors. Prefer using the [`@assert.target`](../services/constraints#assert-target) for application-level input validation, which is more tuned for typical application scenarios, with error messages tailored for end users.
+
+#### `ON DELETE CASCADE`
+
+Think of the example above: a book can still exist, even if its author is deleted. However, this differs for existential relationships like the one between a book and its pages: a page cannot exist without a corresponding book. In such cases, you want the pages to be automatically deleted when the book (parent) is deleted.
+
+Typically such existential relationships are modeled in CDS using [compositions](../../cds/cdl#compositions).
+That is why for managed **backlink** associations (those used in a composition's on-condition via `$self = <comp>.<backlink>`), the foreign key constraints are generated with `ON DELETE CASCADE` by default, instead of `ON DELETE RESTRICT`:
+
+::: code-group
+```cds [CDS Source]
+entity Books {
+  key ID : Integer;
+  pages: Composition of many Pages on pages.book = $self;
+}
+entity Pages {
+  key number: Integer;
+  key book: Association to Books; // → ON DELETE CASCADE
+}
+```
+:::
+
+::: code-group
+```sql [=>  Generated DDL]
+…
+ALTER TABLE Pages ADD CONSTRAINT c__Pages_book
+  FOREIGN KEY(book_ID) REFERENCES Books(ID)
+  ON UPDATE RESTRICT
+  ON DELETE CASCADE
+  VALIDATED
+  ENFORCED
+  INITIALLY DEFERRED;
+```
+:::
 
 #### Skipping with `@assert.integrity:false`