feature #6932 Explain the limitations of the custom messages in UniqueEntity (javiereguiluz)

This PR was submitted for the 2.8 branch but it was merged into the 3.1 branch instead (closes #6932).

Discussion
----------

Explain the limitations of the custom messages in UniqueEntity

This fixes #6929. Thanks goes to @HeahDude, who made all the "detective work" to find this limitation.

Commits
-------

4c10ea8 Explain the limitations of the custom messages in UniqueEntity

Author: wouterj

reference/constraints/UniqueEntity.rst

@@ -132,7 +132,18 @@ message
 
 **type**: ``string`` **default**: ``This value is already used.``
 
-The message that's displayed when this constraint fails.
+The message that's displayed when this constraint fails. This message is always
+mapped to the first field causing the violation, even when using multiple fields
+in the constraint.
+
+.. versionadded:: 3.1
+    The ability to include the invalid value into the message was introduced
+    in Symfony 3.1.
+
+Messages can include the ``{{ value }}`` placeholder to display a string
+representation of the invalid entity. If the entity doesn't define the
+``__toString()`` method, the following generic value will be used: *"Object of
+class __CLASS__ identified by <comma separated IDs>"*
 
 em
 ~~