minor #15012 [Validator] Mention doctrine-bridge in the UniqueEntity constraint (wkania)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] Mention doctrine-bridge in the UniqueEntity constraint

Like in the [UserPassword](https://symfony.com/doc/4.4/reference/constraints/UserPassword.html).
Changed also plural to singular because there is only one constraint in those.

Commits
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a757019 [Validator] Mention doctrine-bridge in the UniqueEntity constraint

Author: javiereguiluz

reference/constraints/UniqueEntity.rst

@@ -10,6 +10,11 @@ using an email address that already exists in the system.
     If you want to validate that all the elements of the collection are unique
     use the :doc:`Unique constraint </reference/constraints/Unique>`.
 
+.. note::
+
+    In order to use this constraint, you should have installed the
+    symfony/doctrine-bridge with Composer.
+
 ==========  ===================================================================
 Applies to  :ref:`class <validation-class-target>`
 Options     - `em`_

reference/constraints/UserPassword.rst

@@ -12,7 +12,7 @@ password, but needs to enter their old password for security.
 
 .. note::
 
-    In order to use this constraints, you should have installed the
+    In order to use this constraint, you should have installed the
     symfony/security-core component with Composer.
 
 ==========  ===================================================================