[documentation]: prompt2prompt needs better docs

[damian0815]

.swap() is a complicated feature with some strange failure modes, which sometimes appears to be doing nothing or the wrong thing.

Documentation should be improved with images so that the use-cases and expected functionality of .swap() are more clear.

[damian0815]

(this issue was raised by @n00mkrad)

[damian0815]

i made some good example images in the pull request where this feature was implemented (something about a cat.swap(dog) eating a hotdog) - should dig these out again.