`string_view::contains` is undocumented

[Flamefire]

The docs do not mention the contains members.

Also ends_with and friends are still marked as "boost extensions":

utility/include/boost/utility/string_view.hpp

Line 244 in e675dfc

BOOST_CONSTEXPR bool ends_with(charT c) const BOOST_NOEXCEPT { // Boost extension

They are part of C++20 std::string_view now.