XTCE13 — Clarify RepeatEntry's values and what they mean

Firstly because repeat is optional, it seems intuitive that this is the same as repeat=0. And that intuitively means the entry repeats 0 times, which means it appears 1 time in the container entry list. Likewise a repeat=1, means the entry appears 2 times and forth. This to me makes sense but maybe it doesn't.
The syntax right now is broad and would allow all sort of values including negative ones. It's not clear that that would be anything but an error.
Given all that we should say something about this in the annotation at the very least. And since FixedValue is a long we could change it to positive-only or perhaps non-negative. The DiscreteLookup value's type could also be modified (we'd have to make a "PositveOnlyDiscreteLookup" or some such). The last is a DynamicLookup and that would have to be enforced outside the syntax, we could imply it by calling it "PositiveOnlyDynamicLookup" or similar.
Finally add annotation.