The following patch tries to address the lack of slot documentation, since getting the slots of a dependency right seems like a common problem.
Things that I was particularly not sure about: the 'subslots' element. Having a sub-element for 'slot' seemed even more messy, so I tried to make this as simple as possible, so that maintainers don't get angry and give up when trying to document their slots. However, this is a little bit at the expense of correctness, because you cannot document different subslot naming schemes if they differ between slots of a single package (does such thing even exist in the tree?).
