Document getbbox change for images with alpha [ci skip]

docs/releasenotes/7.1.0.rst

@@ -14,3 +14,12 @@ been resolved.
     from PIL import Image
     im = Image.open("hopper.jpg")
     im.save("out.jpg", quality=0)
+
+If present, only use alpha channel for bounding box
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When the :py:meth:`~PIL.Image.Image.getbbox` method calculates the bounding
+box, for an RGB image it trims black pixels. Similarly, for an RGBA image it
+would trim black transparent pixels. This is now changed so that if an image
+has an alpha channel (RGBA, RGBa, PA, LA, La), any transparent pixels are
+trimmed.