Merge pull request #6831 from radarhere/transparency_documentation

Document "transparency" info key

docs/handbook/concepts.rst

@@ -111,6 +111,18 @@ the file format handler (see the chapter on :ref:`image-file-formats`). Most
 handlers add properties to the :py:attr:`~PIL.Image.Image.info` attribute when
 loading an image, but ignore it when saving images.
 
+Transparency
+------------
+
+If an image does not have an alpha band, transparency may be specified in the
+:py:attr:`~PIL.Image.Image.info` attribute with a "transparency" key.
+
+Most of the time, the "transparency" value is a single integer, describing
+which pixel value is transparent in a "1", "L", "I" or "P" mode image.
+However, PNG images may have three values, one for each channel in an "RGB"
+mode image, or can have a byte string for a "P" mode image, to specify the
+alpha value for each palette entry.
+
 Orientation
 -----------