[Validator] UniqueEntity - add warning about not supported types and association mappings.

This constraint does not support Doctrine types such as simple_array, json, or jsonb, nor does it handle association mappings like One-To-Many and Many-To-Many relationships. To perform uniqueness checks on such fields, you must define a custom repositoryMethod that implements the desired logic.

Author: wkania

reference/constraints/UniqueEntity.rst

@@ -402,4 +402,12 @@ blank, ``findBy()`` will be used. The method receives as its argument a
 fields configured in the ``fields`` option). The method should return a
 :phpfunction:`countable PHP variable <is_countable>`.
 
+.. warning::
+
+    This constraint does not support Doctrine types such as ``simple_array``, 
+    ``json``, or ``jsonb``, nor does it handle association mappings like 
+    ``One-To-Many`` and ``Many-To-Many`` relationships. To perform uniqueness
+    checks on such fields, you must define a custom ``repositoryMethod`` that
+    implements the desired logic.
+
 .. _`race conditions`: https://en.wikipedia.org/wiki/Race_condition