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

[alandefreitas]

This has fixed itself via other PRs that include features to exclude symbols from the documentation without macros. We're not going to follow the Doxygen strategy of trying to handle ill-formed code.