Clarify handling scores greater than 255 in allergies

[asarkar]

I found several closed tickets related to this after my code failed the last test. The following comment is anything but clear. The first thought is to take a modulo, but the test expects a bitwise-AND operation.

For example, if the allergy score is 257, your program should only report the eggs (1) allergy.

Please add some explanation as to why, may be something like this comment.

#224
#389
exercism/rust#209