math/big: correct documentation for ProbablyPrime.
As akalin points out in the bug, the comment previously claimed that the
probability that the input is prime given that the function returned
true is 1 - ¼ⁿ. But that's wrong: the correct statement is that the
probability of the function returning false given a composite input is
1 - ¼ⁿ.
This is not nearly as helpful, but at least it's truthful. A number of
other (correct) expressions are suggested on the bug, but I think that
the simplier one is preferable.
This change also notes that the function is not suitable for
adversarial inputs since it's deterministic.