add omitted ImageFilter types

docs/reference/ImageFilter.rst

@@ -66,3 +66,29 @@ image enhancement filters:
 
 .. autoclass:: PIL.ImageFilter.ModeFilter
     :members:
+
+.. class:: Filter
+
+    An abstract mixin used for filtering images
+    (for use with :py:meth:`~PIL.Image.Image.filter`)
+
+    Implementors must provide the following method:
+
+    .. method:: filter(self, image)
+
+        Applies a filter to a single-band image, or a single band of an image.
+
+        :returns: A filtered copy of the image.
+
+.. class:: MultibandFilter
+
+    An abstract mixin used for filtering multi-band images
+    (for use with :py:meth:`~PIL.Image.Image.filter`)
+
+    Implementors must provide the following method:
+
+    .. method:: filter(self, image)
+
+        Applies a filter to a multi-band image.
+
+        :returns: A filtered copy of the image.

docs/releasenotes/4.3.0.rst

@@ -124,7 +124,7 @@ This release contains several performance improvements:
 * ``Image.transpose`` has been accelerated 15% or more by using a cache
   friendly algorithm.
 * ImageFilters based on Kernel convolution are significantly faster
-  due to the new MultibandFilter feature.
+  due to the new :py:class:`~PIL.ImageFilter.MultibandFilter` feature.
 * All memory allocation for images is now done in blocks, rather than
   falling back to an allocation for each scan line for images larger
   than the block size.