Documented default quantization method

docs/reference/Image.rst

@@ -486,15 +486,15 @@ Used to specify the quantization method to use for the :meth:`~Image.quantize` m
 
 .. data:: MEDIANCUT
 
-    Median cut
+    Median cut. Default method, except for RGBA images.
 
 .. data:: MAXCOVERAGE
 
-    Maximum coverage
+    Maximum coverage.
 
 .. data:: FASTOCTREE
 
-    Fast octree
+    Fast octree. Default method for RGBA images.
 
 .. data:: LIBIMAGEQUANT
 

src/PIL/Image.py

@@ -1059,6 +1059,11 @@ class Image:
                        :data:`LIBIMAGEQUANT` (libimagequant; check support using
                        :py:func:`PIL.features.check_feature`
                        with ``feature="libimagequant"``).
+
+                       By default, :data:`MEDIANCUT` will be used.
+
+                       The exception to this is RGBA images. RGBA images use
+                       :data:`FASTOCTREE` by default instead.
         :param kmeans: Integer
         :param palette: Quantize to the palette of given
                         :py:class:`PIL.Image.Image`.