doc/annotation: add cascade deletion doc and example (#1340)

doc/md/schema-annotations.md

@@ -42,4 +42,46 @@ func (User) Fields() []ent.Field {
 		field.String("name"),
 	}
 }
-```  
+```
+
+## Foreign Keys Configuration
+
+Ent allows to customize the foreign key creation and provide a [referential action](https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html#foreign-key-referential-actions)
+for the `ON DELETE` clause:
+
+```go
+package schema
+
+import (
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+)
+
+// User holds the schema definition for the User entity.
+type User struct {
+	ent.Schema
+}
+
+// Fields of the User.
+func (User) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("name").
+			Default("Unknown"),
+	}
+}
+
+// Edges of the User.
+func (User) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("posts", Post.Type).
+			Annotations(entsql.Annotation{
+				OnDelete: entsql.Cascade,
+			}),
+	}
+}
+```
+
+The example above configures the foreign key to cascade the deletion of rows in the parent table to the matching
+rows in the child table.