Fix docs on final entities (doctrine#12176)

Author: garak

docs/en/reference/advanced-configuration.rst

@@ -76,6 +76,8 @@ Configuration Options
 The following sections describe all the configuration options
 available on a ``Doctrine\ORM\Configuration`` instance.
 
+.. _reference-native-lazy-objects:
+
 Native Lazy Objects (**OPTIONAL**)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

docs/en/reference/architecture.rst

@@ -79,8 +79,9 @@ Entities
 An entity is a lightweight, persistent domain object. An entity can
 be any regular PHP class observing the following restrictions:
 
--  An entity class must not be final nor read-only but
-   it may contain final methods or read-only properties.
+-  An entity class can be final or read-only when
+   you use :ref:`native lazy objects <reference-native-lazy-objects>`.
+   It may contain final methods or read-only properties too.
 -  Any two entity classes in a class hierarchy that inherit
    directly or indirectly from one another must not have a mapped
    property with the same name. That is, if B inherits from A then B

docs/en/tutorials/getting-started.rst

@@ -49,8 +49,9 @@ An entity contains persistable properties. A persistable property
 is an instance variable of the entity that is saved into and retrieved from the database
 by Doctrine's data mapping capabilities.
 
-An entity class must not be final nor read-only, although
-it can contain final methods or read-only properties.
+An entity class can be final or read-only when you use
+:ref:`native lazy objects <reference-native-lazy-objects>`.
+It may contain final methods or read-only properties too.
 
 An Example Model: Bug Tracker
 -----------------------------