Clarified that the sequence object for putdata() should be flattened

src/PIL/Image.py

@@ -1707,13 +1707,14 @@ def putalpha(self, alpha):
 
     def putdata(self, data, scale=1.0, offset=0.0):
         """
-        Copies pixel data to this image.  This method copies data from a
-        sequence object into the image, starting at the upper left
-        corner (0, 0), and continuing until either the image or the
-        sequence ends.  The scale and offset values are used to adjust
-        the sequence values: **pixel = value*scale + offset**.
-
-        :param data: A sequence object.
+        Copies pixel data from a flattened sequence object into the image. The
+        values should start at the upper left corner (0, 0), continue to the
+        end of the line, followed directly by the first value of the second
+        line, and so on. Data will be read until either the image or the
+        sequence ends. The scale and offset values are used to adjust the
+        sequence values: **pixel = value*scale + offset**.
+
+        :param data: A flattened sequence object.
         :param scale: An optional scale value.  The default is 1.0.
         :param offset: An optional offset value.  The default is 0.0.
         """