Ignored classes are being documented

[alandefreitas]

This issue has been split from #480 (Features missing for Boost.URL) as it turned out to be more complex than we initially thought.

Classes that are ifdeffed out are being documented: https://792.urlantora.prtest2.cppalliance.org/site/url/reference/boost/urls/ignore_case_t.html / https://github.com/boostorg/url/blob/cf2a888c42dedc25f8223dc61d4707e83bd7a22b/include/boost/url/ignore_case.hpp#L18-L22

See solution for #583