transaction: document `tr.add`

transaction: drop backupentries logic from startgroup and endgroup The `startgroup` and `endgroup` methods are used in a very specific context to wrap a very specific operation (revlog truncation). It does not make sense to perform any other operations during such a "group" (eg:file backup). There is currently no user of backupfile during a "group" so we drop the group-specific code and restrict authorized operations during "group".

transaction: document startgroup and endgroup These enigmatic methods are only used in repair. We document them to clarify there purpose and user.

transaction: mark backup-related attributes private As the transaction is gaining more functions and attributes, it is important to clarify what is part of the public API.