bpo-33283: Mention PNG as a supported format by Tcl/Tk. (GH-6479) (GH-6542)

(cherry picked from commit 4b685bf)

Update only the documentation about image formats.

Author: andresdelfino

Doc/library/tkinter.rst

@@ -801,12 +801,13 @@ Menu indexes (menu.invoke(), menu.entryconfig(), etc.)
 Images
 ^^^^^^
 
-Bitmap/Pixelmap images can be created through the subclasses of
-:class:`Tkinter.Image`:
+Images of different formats can be created through the corresponding subclass
+of :class:`Tkinter.Image`:
 
-* :class:`BitmapImage` can be used for X11 bitmap data.
+* :class:`BitmapImage` for images in XBM format.
 
-* :class:`PhotoImage` can be used for GIF and PPM/PGM color bitmaps.
+* :class:`PhotoImage` for images in PGM, PPM, GIF and PNG formats. The latter
+  is supported starting with Tk 8.6.
 
 Either type of image is created through either the ``file`` or the ``data``
 option (other options are available as well).

Lib/lib-tk/Tkinter.py

@@ -3362,7 +3362,7 @@ def width(self):
             self.tk.call('image', 'width', self.name))
 
 class PhotoImage(Image):
-    """Widget which can display colored images in GIF, PPM/PGM format."""
+    """Widget which can display images in PGM, PPM, GIF, PNG format."""
     def __init__(self, name=None, cnf={}, master=None, **kw):
         """Create an image with NAME.
 
@@ -3426,7 +3426,7 @@ def write(self, filename, format=None, from_coords=None):
         self.tk.call(args)
 
 class BitmapImage(Image):
-    """Widget which can display a bitmap."""
+    """Widget which can display images in XBM format."""
     def __init__(self, name=None, cnf={}, master=None, **kw):
         """Create a bitmap with NAME.