It’s not documented, but it’s not horrible to read. LBRN is simpler - LBRN2 was written to be more compact, and as such, it’s less readable. LBRN was written to be human readable so that people would be able to hack / create them. It’s just XML, and most of it should be easy enough to work out by saving a couple of examples.
The xform will probably require explanation, but have a look through an LBRN file or two, and report back if you don’t understand what something is and we can fill in the blanks for you.