Update filter group docs

django_filters/groups.py

@@ -1,5 +1,6 @@
 import functools
 import operator
+from abc import abstractmethod
 
 from django.core.exceptions import ValidationError
 from django.db.models import Q
@@ -30,6 +31,7 @@ def __init__(self, filters):
         self.parent = None
         self.model = None
 
+    @abstractmethod
     def validate(self, form, **data):
         """Validate the subset of cleaned data provided by the form.
 
@@ -42,13 +44,17 @@ def validate(self, form, **data):
         """
         raise NotImplementedError
 
+    @abstractmethod
     def filter(self, qs, **data):
         """Filter the result queryset with the subset of cleaned data.
 
         Args:
             qs: The ``QuerySet`` instance to filter.
             **data: The subset of a form's ``cleaned_data`` for the filters in
                 the group.
+
+        Returns:
+            The filtered queryset instance.
         """
         raise NotImplementedError
 

docs/ref/groups.txt

@@ -3,10 +3,9 @@ Group Reference
 ===============
 
 By default, a ``FilterSet`` validates and then filters its query parameters
-independently of each other, ultimately resulting in a sequence of chained
-``.filter()`` calls. Due to the nature of filter chaining, it's not trivial
-to, for example, make a group of filters mutually exclusive, or to OR them
-together into a single filter call.
+independently of each other, ultimately resulting in a chain of ``.filter()``
+calls. Due to the nature of filter chaining, it's not trivial to, for example,
+make a group of filters that are mutually exclusive to each other.
 
 Filter groups solve this limitation by processing subsets of parameters
 together, enabling higher level validation and filtering behavior.
@@ -15,9 +14,9 @@ Validation
 ==========
 
 A filter group only applies group-level validation. For example, a mutually
-exclusive group is only responsible for validating that one of its parameters
-is present. It is not responsible for individual value validation, which is
-still performed by the underlying filter instances.
+exclusive group is responsible for validating that *only* one of its parameters
+is present. It is not responsible for validating the individual values, which
+is still performed by the underlying filter instances.
 
 Filtering
 =========