Symbols not being documented in Boost.URL

[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.

mrdocs doesn't pick up the javadocs for many classes (https://792.urlantora.prtest2.cppalliance.org/site/url/reference/boost/urls/ignore_case.html). Probably because they have some ifdefs involved (https://github.com/boostorg/url/blob/cf2a888c42dedc25f8223dc61d4707e83bd7a22b/include/boost/url/ignore_case.hpp#L31-L37).

The problem here is symbols in header files that end up not being transitively included by source files are not documented because of how we use the compile_commands.json file. standardese proposes a solution to this and doxygen also has a different solution to this problem. We can compare what these other tools do here.

[alandefreitas]

This has fixed itself via other PRs that included the documentation for classes and functions.