minor #4752 [Book][Validation] clarify group validation (xabbuh)

This PR was merged into the 2.5 branch.

Discussion
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[Book][Validation] clarify group validation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | part of #4094

This is for item `F` of #4094. Actually, we didn't have anything wrong as long as I didn't miss anything. I only made two little changes to fix a sentence and clarify the legacy API.

Commits
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1c79cad clarify group validation

book/validation.rst

@@ -856,10 +856,10 @@ as the third argument to the ``validate()`` method::
     // If you're using the new 2.5 validation API (you probably are!)
     $errors = $validator->validate($author, null, array('registration'));
 
-    // If you're using the old 2.4 validation API
+    // If you're using the old 2.4 validation API, pass the group names as the second argument
     // $errors = $validator->validate($author, array('registration'));
 
-If no groups are specified, all constraints that belong in group ``Default``
+If no groups are specified, all constraints that belong to the group ``Default``
 will be applied.
 
 Of course, you'll usually work with validation indirectly through the form