I'll attempt to answer your question ...
None of which I am aware.
There is this in official Moodle docs:
https://docs.moodle.org/dev/Backup_2.0_general_architecture so old one should not use it. It has cvs links to a code repo long since removed as Moodle has gone to git. *see below*
And an old related discussion:
https://moodle.org/mod/forum/discuss.php?d=121595
which is gonna say the same thing I'll say here.
Bottom line: no - there are no docs.
If one extracts a backup one can study the .xml files and their related directory.
* IF one could find a git tree of the code you might be able to inspect
current code for backup routines and thus, might discover what you need.
* git hub:
https://github.com/moodle/moodle/tree/master/backup
'spirit of sharing', Ken