[cmp, comparisons] Move description of compare_three_way.

Moves the definition and specification of class compare_three_way from [cmp, 17.11] to [comparisons, 20.14.7]. Also adds a declaration of this class to the synopsis of and removes a paragraph that says (now redundantly) that the class is available also via inclusion of .

This is a partial response to NB GB 175 (C++20 CD).