Document using encoderinfo on subsequent frames from python-pillow#8483

src/PIL/Image.py

@@ -2475,7 +2475,21 @@ def save(
            format to use is determined from the filename extension.
            If a file object was used instead of a filename, this
            parameter should always be used.
-        :param params: Extra parameters to the image writer.
+        :param params: Extra parameters to the image writer. These can also be
+           set on the image itself through ``encoderinfo``. This is useful when
+           saving multiple images::
+
+             # Saving XMP data to a single image
+             from PIL import Image
+             red = Image.new("RGB", (1, 1), "#f00")
+             red.save("out.mpo", xmp=b"test")
+
+             # Saving XMP data to the second frame of an image
+             from PIL import Image
+             black = Image.new("RGB", (1, 1))
+             red = Image.new("RGB", (1, 1), "#f00")
+             red.encoderinfo = {"xmp": b"test"}
+             black.save("out.mpo", save_all=True, append_images=[red])
         :returns: None
         :exception ValueError: If the output format could not be determined
            from the file name.  Use the format option to solve this.