Mention that tobytes() with the raw encoder uses Pack.c

src/PIL/Image.py

@@ -767,18 +767,20 @@ def tobytes(self, encoder_name: str = "raw", *args: Any) -> bytes:
 
         .. warning::
 
-            This method returns the raw image data from the internal
-            storage.  For compressed image data (e.g. PNG, JPEG) use
-            :meth:`~.save`, with a BytesIO parameter for in-memory
-            data.
-
-        :param encoder_name: What encoder to use.  The default is to
-                             use the standard "raw" encoder.
-
-                             A list of C encoders can be seen under
-                             codecs section of the function array in
-                             :file:`_imaging.c`. Python encoders are
-                             registered within the relevant plugins.
+            This method returns raw image data derived from Pillow's internal
+            storage. For compressed image data (e.g. PNG, JPEG) use
+            :meth:`~.save`, with a BytesIO parameter for in-memory data.
+
+        :param encoder_name: What encoder to use.
+
+                             The default is to use the standard "raw" encoder.
+                             To see how this packs pixel data into the returned
+                             bytes, see :file:`libImaging/Pack.c`.
+
+                             A list of C encoders can be seen under codecs
+                             section of the function array in
+                             :file:`_imaging.c`. Python encoders are registered
+                             within the relevant plugins.
         :param args: Extra arguments to the encoder.
         :returns: A :py:class:`bytes` object.
         """