changed documentation of isprime(n::Integer) -> Bool (#147)

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

---------

Co-authored-by: fepaul <fepaul@students.uni-mainz.de>

Author: fepaul-book

src/Primes.jl

@@ -156,8 +156,14 @@ end
 """
     isprime(n::Integer) -> Bool
 
-Returns `true` if `n` is prime, and `false` otherwise.
-
+Returns for values in the range of an INT64 variable:  `true` if `n` is prime, and `false` otherwise 
+        for bigger values: `true` if `n` is probably prime, and `false` otherwise (false-positive rate = 0.25^reps with reps=25 --> considerered safe)
+
+    More detailed:
+    for even numbers: returns deterministic and correct results
+    for values in the range of an  INT64 variable: returns deterministic and correct results (by Lookup-tables, trial-division, Miller-Rabin, Lucas-Test)
+    for bigger values: returns probabilistic resultsfrom GNU Multiple Precision Arithmetic Library 
+    
 ```julia
 julia> isprime(3)
 true
@@ -290,6 +296,9 @@ end
 Probabilistic primality test. Returns `true` if `x` is prime with high probability (pseudoprime);
 and `false` if `x` is composite (not prime). The false positive rate is about `0.25^reps`.
 `reps = 25` is considered safe for cryptographic applications (Knuth, Seminumerical Algorithms).
+
+is_probably_prime is inherited from the module IntegerMathUtils that provides a wrapper to access
+functionality from the  GNU Multiple Precision Arithmetic Library (GMP) library
 ```julia
 julia> isprime(big(3))
 true