Review alias handlebars templates

[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.

Aliases are documented with the alias itself on the right-hand side: using error_category = error_category; (now they are using error_category = system::error_category;, which is not relative to the current alias namespace as in https://www.boost.org/doc/libs/master/libs/url/doc/html/url/ref/boost__urls__error_category.html.

I don't know if this has been completely fixed by @fpelliccioni's changes but we have to compare that with whatever doxygen does here.