Re-add the API to get the document where the wiki descriptor is stored

The APIs to handle subwikis were refactored in XWiki 5.3 and the subwikis creation was integrated in the core, but with this refactoring the API that allowed to access the document where a Wiki descriptor is stored was lost.
Right now, to access that document we need to compose its name manually from the id of the wiki (thus duplicating the code of the wiki manager which does the same thing internally).

One counter-argument to having this API back is the fact that the descriptor document is only an implementation detail and, as long as we can access all the metadata about a Wiki, we don't need to know that they are stored in a document.
But I think the document where the wiki descriptor is setup is interesting enough and stable enough to be part of the API. It is useful for adding a layer of data related to a subwiki that is specific to a certain usage of the subwiki (e.g. storing a categorisation of wikis, or making a hierarchy of wikis and storing it as a parent-child hierarchy of the descriptors).