changed documentation of isprime(n::Integer) -> Bool

[fepaul-book]

• shows exactly when probabilistic methods are applied and when not
• mentions the used methods

changed documentation of isprime(x::BigInt, [reps = 25]) -> Bool

• stated where the function is_probably_prime comes from and explains what it does

[oscardssmith]

Thanks!

[fepaul-book]

cc @ranocha