Add documentation for DeprecatedFFIPrime warnings

[kl0tl]

This PR documents the warning that will be emitted for foreign identifiers containing primes once purescript/purescript#3792 is merged.

[hdgarrood]

Thanks! This is a good start, but I think it could be made clearer, by a) putting a ' in brackets in the "cause" sentence so that it's clearer what a prime character is, like you did in the error message in the compiler repo, and b) showing more explicitly how to update the code to silence the warning, i.e. reproducing the example except without the primes.

[kl0tl]

This looks indeed clearer with your suggestions!

[hdgarrood]

Looks great 👍