minor #10409 Added a tip about atPath() method (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #10409).

Discussion
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Added a tip about atPath() method

Fixes #5049.

Commits
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5c00a65 Added a tip about atPath() method

Author: javiereguiluz

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

@@ -3,13 +3,10 @@ property_path
 
 **type**: ``any`` **default**: ``the field's name``
 
-Fields display a property value of the form's domain object by default.
-When the form is submitted, the submitted value is written back into the
-object.
-
-If you want to override the property that a field reads from and writes
-to, you can set the ``property_path`` option. Its default value is the field's
-name.
+By default form fields read from and write to the properties with the same names
+in the form's domain object. The ``property_path`` option lets you define which
+property a field reads from and writes to. The value of this option can be any
+:doc:`valid PropertyAccess syntax </components/property_access>`.
 
 If you wish the field to be ignored when reading or writing to the object
 you can set the ``property_path`` option to ``false``, but using

validation/custom_constraint.rst

@@ -262,6 +262,12 @@ With this, the validator ``validate()`` method gets an object as its first argum
         }
     }
 
+.. tip::
+
+    The ``atPath()`` method defines the property which the validation error is
+    associated to. Use any :doc:`valid PropertyAccess syntax </components/property_access>`
+    to define that property.
+
 Note that a class constraint validator is applied to the class itself, and
 not to the property: