Improve Data.Functor documentation

[chessai .]

A beginner learning about parametricity and the relationship between
Functorial composition and Functor identity brought up a good point to me:
the documentation for Bifunctor[1] and Profunctor[2] both explain the
relationship between parametricity and their laws, particularly what
constitutes a minimum "burden" of proof. I don't have an exact wording yet,
but I think it would be a good idea to update the Functor documentation
accordingly.

A separate point of issue: The line "The instances of Functor for lists,
Maybe and IO satisfy these laws." This statement is redundant at best when
you consider that Haskellers aim for instances of Functor to be lawful. At
worst it could leave beginners confused and with more questions. I could
imagine someone interpreting this statement as saying there might exist
some unlawful instances. I think this line should just be removed.

Let me know what you think.

Thanks
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