Document that the nested ActiveModel builder requires the dense entity format (#2879)

Adds a note to the nested object-graph builder example (README and crate docs) that it requires the SeaORM 2.0 dense entity format.

Author: Huliiiiii

README.md

@@ -143,6 +143,7 @@ smart_user
 Persist an entire object graph: user, profile (1-1), posts (1-N), and tags (M-N)
 in a single operation using a fluent builder API. SeaORM automatically determines
 the dependencies and inserts or deletes objects in the correct order.
+This requires the SeaORM 2.0 dense entity format.
 
 ```rust
 // this creates the nested object as shown above:

src/lib.rs

@@ -217,6 +217,7 @@
 //! Persist an entire object graph: user, profile (1-1), posts (1-N), and tags (M-N)
 //! in a single operation using a fluent builder API. SeaORM automatically determines
 //! the dependencies and inserts or deletes objects in the correct order.
+//! This requires the SeaORM 2.0 dense entity format.
 //!
 //! ```
 //! # use sea_orm::{DbConn, DbErr, entity::*, query::*, tests_cfg::*};