I find it most helpful when the header synopsis lists all top-level inhabitants of namespace std.

Would that include deduction guides? We notably do not depict the declarations of deduction guides in the header synopses, they are conventionally declared together with the corresponding class template synopsis. (Which fact I personally have always found odd.)

Should our conventions ensure that all namespace-scope declarations (except those which exist only for exposition) appear in header synopses?

For modules, deduction guides are "invisible" (in the sense that we don't/can't directly export them, they're just associated with the primary template in some magical way). There may still be reasons for an editorial preference to put them in the header synopsis, but seeing what's in the std module wouldn't be such a reason.

Similar example (I can file a separate issue if the editors want):

[ranges.syn] depicts only:

// 26.5.4, sub-ranges
enum class subrange_kind : bool { unsized, sized };

template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = see below >
  requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
class subrange;

template<class I, class S, subrange_kind K>
  inline constexpr bool enable_borrowed_range<subrange<I, S, K>> = true;

However, [range.subrange.general] defines more:

namespace std::ranges {
  // [...exposition only machinery...]

  template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
      sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
    requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
  class subrange : public view_interface<subrange<I, S, K>> {
    // [...class definition...]
  };
  
  // [...deduction guides...]
  
  template<size_t N, class I, class S, subrange_kind K>
    requires ((N == 0 && copyable<I>) || N == 1)
    constexpr auto get(const subrange<I, S, K>& r);
  
  template<size_t N, class I, class S, subrange_kind K>
    requires (N < 2)
    constexpr auto get(subrange<I, S, K>&& r);
}

namespace std {
  using ranges::get;
}

I expected to see get(const subrange&), get(subrange&&), and using ranges::get; appear in the <ranges> synopsis.

The current operative rule is that deduction guides go with the class synopsis; any other namespace-scope declaration (which need not be a definition) is shown in the header synopsis.