feature #6156 [reference] [form] [options] fix #6153 (HeahDude)

This PR was squashed before being merged into the 2.3 branch (closes #6156).

Discussion
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[reference] [form] [options] fix #6153

| Q             | A
| ------------- | ---
| Doc fix?        | yes
| New docs?   | no
| Applies to     | 2.3+
| Fixed tickets | #6153

Commits
-------

a15f856 [reference] [form] [options] fix #6153

Author: wouterj

reference/forms/types/options/error_mapping.rst.inc

@@ -34,7 +34,14 @@ Here are the rules for the left and the right side of the mapping:
   object, the property path is ``[indexName]``;
 * You can construct nested property paths by concatenating them, separating
   properties by dots. For example: ``addresses[work].matchingCityAndZipCode``;
-* The left side of the error mapping also accepts a dot ``.``, which refers
-  to the field itself. That means that any error added to the field is added
-  to the given nested field instead;
 * The right side contains simply the names of fields in the form.
+
+Additionally, you can set the left side to dot (``.``), which refers to any
+unmapped property or method needing validation to the given nested field instead
+of bubbling them to the form::
+
+    $resolver->setDefaults(array(
+        'error_mapping' => array(
+            '.' => 'city',
+        ),
+    ));