Backpack documentation is lacking

Since its merge Backpack has been long lacking in the documentation department. In particular, there are several questions of the implementation that are currently unaddressed in the codebase:

• What is the motivation for and semantics of the .bkp file format?
• How do Cabal (and other build systems) interact with
• Where are the Notes that do discuss Backpack found?
• How does Backpack interact with the EPS? How are hole instantiations incorporated into UnitIds?
• What shortcomings does the current implementation suffer from? For instance, until very recently (#17188 (closed)) -j was not supported under Backpack. There are also known limitations regarding promotion (#13149). Do we have a comprehensive list of such limitations somewhere (perhaps in the user documentation)?

Furthermore, the users guide has not a single mention of the feature, its motivation, or user-facing interface.