Update docs (#31)
Update docs (#31)

See also #23, #29

I thought about this a bit more, and I think the reasoning for why refs
are effectful is more persuasive when viewed through the lens of purity
rather than of referential transparency, so I've updated the docs
accordingly.

I am reluctant to include code examples illustrating why Refs are
effectful here, because I think people may skim the page looking for
code examples, and in the absence of code examples which describe the
real API, I think code examples which illustrate a hypothetical version
of the API which doesn't actually exist may do more harm than good.

I am also leaning towards not including @natefaubion's example (where
you can violate the type system by defining a reference with a
polymorphic type and specializing it after the fact), since it's hard to
illustrate without a code example, and I think it's unlikely to be
useful to most people who just want to make use of the Ref API, which is
the group I think we should be considering to be our target audience
here.
purescript/purescript-refs