mpz_nextprime documentation

[keith.briggs at bt.com]

http://www.swox.com/gmp/manual/Number-Theoretic-Functions.html#Number%20Theoretic%20Functions:
> Set rop to the next prime greater than op. 
>This function uses a probabilistic algorithm to identify primes. For practical purposes it's adequate, the chance of a composite passing will be extremely small. 
It would be nice if it told us if there's any chance of it missing a prime!

Keith