Slight improvement to the docs

Fix few warnings when building the docs (the others being false-positive).

Also, what's the way to deal with the `$(D ...)` macro ?
It doesn't seem to be documented in http://dlang.org/ddoc.html
I thought it was for D keyword, but `source.vibe.http.server` L206 use it for an XREF. So how should it be used (as it's currently inconsistent in the doc).