[ 2021-05-19 Tim updates wording ]

The wording below removes the explicit precondition on the sized_sentinel_for overload of distance, relying instead on the semantic requirements of that concept and the "Effects: Equivalent to:" word of power. This also removes the potentially surprising inconsistency that given a non-empty std::vector<int> v, ranges::distance(v.begin(), v.cend()) is well-defined but ranges::distance(v.cend(), v.begin()) is currently undefined.