Unclear "15" in doc for mpz_probab_prime_p

[Paul Eggert]

Currently gmp.texi says this about mpz_probab_prime_p:

"This function performs some trial divisions, a Baillie-PSW probable 
prime test, then @var{reps-24} Miller-Rabin probabilistic primality 
tests.... Reasonable values of @var{reps} are between 15 and 50."

That "15" is confusing because all reps values < 25 behave the same.

The simplest fix I can think of is to change "15" to "24" in the 
documentation. Alternatively we could add an explanation for the "15"; I 
assume it's for portability to GMP 6.1.2 (2016) and earlier, but it's 
not clear that the extra wording would be worth the trouble nowadays.

Also, two lines earlier in gmp.texi I suggest changing "A composite 
number" to "A randomly chosen composite number" to emphasize that GMP 
doesn't guarantee resistance to large composite numbers that were 
generated by an adversary.