P3688 Eliminate uses of `u32string_view` in wording

[eisenwave]

The use of u32string_view in wording may suggest to the reader that <string_view> is included by <ascii>, but that's not the intention. A potentially better way to word those functions using it would be:

Returns: true if static_cast<char32_t>(c) equals one of U'!', U'"', U'#', ...; false otherwise.