Alternative to #8777

In that PR, a user read https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.tobytes

and interpreted it as "tobytes() will return data exactly matching Pillow's internal storage".

This isn't always true for the raw encoder, and it definitely isn't true for the other encoders.

I've tried to clarify that here by updating the documentation to say that the method returns "raw image data derived from Pillow's internal storage". I've also mentioned Pack.c as the user suggested.