Add documentation for DeprecatedFFIPrime warnings - #293

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.

This looks indeed clearer with your suggestions!

Looks great 👍