minor #7249 [Validator] Fixed wrong inheritance information (peterrehm)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #7249).

Discussion
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[Validator] Fixed wrong inheritance information

There is wrong information about the inheritance of validation groups.

I have tested it extensively and some findings are documented here: symfony/symfony#20857

I also asked Bernhard about what the intended behavior is, but I assume
as I have documented it now was the original expected behaviour.

Commits
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49dfdf4 Fixed wrong inheritance information

Author: xabbuh

validation/groups.rst

@@ -166,11 +166,11 @@ the class name or the string ``Default``.
     object that's actually the one being validated.
 
     If you have inheritance (e.g. ``User extends BaseUser``) and you validate
-    with the class name of the subclass (i.e. ``User``), then all constraints
-    in the ``User`` and ``BaseUser`` will be validated. However, if you
-    validate using the base class (i.e. ``BaseUser``), then only the default
-    constraints in the ``BaseUser`` class will be validated.
-
+    with the class name of the subclass (i.e. ``User``), then only constraints
+    in the ``User`` will be validated. To validate the parent constraints as 
+    well you need to provide multiple groups (i.e ``User`` and ``BaseUser``) or
+    ``Default``.
+    
 To tell the validator to use a specific group, pass one or more group names
 as the third argument to the ``validate()`` method::