[1.5.x] Removed a mention of `Form._errors` from the documentation.

Also removed a sentence that was incorrect: raising a
`ValidationError` inside `Form.clean` doesn't clear the
`cleaned_data` attribute.

Thanks to loic84 and timograham for the review.

Backport of 9aa6d4bdb6 and
0048ed77c7 from master.

docs/ref/forms/validation.txt

@@ -82,6 +82,10 @@ overridden:
   cleaned data if you override this method (by default, ``Form.clean()``
   just returns ``self.cleaned_data``).
 
+  Since the field validation method have been run by the time ``clean()`` is
+  called, you also have access to the form's ``errors`` attribute which
+  contains all the errors raised by previous steps.
+
   Note that any errors raised by your ``Form.clean()`` override will not
   be associated with any field in particular. They go into a special
   "field" (called ``__all__``), which you can access via the
@@ -98,7 +102,8 @@ These methods are run in the order given above, one field at a time.  That is,
 for each field in the form (in the order they are declared in the form
 definition), the ``Field.clean()`` method (or its override) is run, then
 ``clean_<fieldname>()``. Finally, once those two methods are run for every
-field, the ``Form.clean()`` method, or its override, is executed.
+field, the ``Form.clean()`` method, or its override, is executed, no matter if
+the previous methods have raised errors or not.
 
 Examples of each of these methods are provided below.
 
@@ -107,15 +112,6 @@ field, if the ``Field.clean()`` method raises a ``ValidationError``, any
 field-specific cleaning method is not called. However, the cleaning methods
 for all remaining fields are still executed.
 
-The ``clean()`` method for the ``Form`` class or subclass is always run. If
-that method raises a ``ValidationError``, ``cleaned_data`` will be an empty
-dictionary.
-
-The previous paragraph means that if you are overriding ``Form.clean()``, you
-should iterate through ``self.cleaned_data.items()``, possibly considering the
-``_errors`` dictionary attribute on the form as well. In this way, you will
-already know which fields have passed their individual validation requirements.
-
 .. _described later:
 
 Form subclasses and modifying field errors