[libc++] Update the commented "synopsis" in <algorithm> to match current reality.

The synopsis now reflects what's implemented. It does NOT reflect
all of what's specified in C++20. The "constexpr in C++20" markings
are still missing from these 12 algorithms, because they are still
unimplemented by libc++:

    reverse partition sort nth_element next_permutation prev_permutation
    push_heap pop_heap make_heap sort_heap partial_sort partial_sort_copy

All of the above algorithms were excluded from [P0202].

All of the above algorithms were made constexpr in [P0879] (along with
swap_ranges, iter_swap, and rotate — we've already implemented those three).

Differential Revision: https://reviews.llvm.org/D92255