As said by Florian (thanks to him), a documentation exists for this
extension point. This documentation is not yet in Sirius, because as
said by Pierre-Charles in this thread ,
> "we used to have a "Developer" section in the documentation (before the product was open sourced as Sirius), but it was inconsistent and sometimes completely obsolete. We did not have enough time to rewrite it for Sirius 1.0, so it has been removed to avoid confusion.
> We certainly want to have such a documentation in the future. Feel free to comment on https://bugs.eclipse.org/bugs/show_bug.cgi?id=426583 to tell us what kind of information you expect in priority for such a document (for us who have been working on Sirius for years, it is not always obvious what a newcomer needs).