Improve docs for AVFrame::image_copy_to_buffer and AVFrame::image_get_buffer_size

[Dryvnt]

Thank you for quickly merging my earlier PR and releasing it. Looking at it again with fresh eyes, I think the doc comments could have been more helpful, especially because it can be confusing what "align" means in this context if you don't already know.

Here is my attempt at making them more helpful, including illustrative examples.