Would reading some proposed #FreeBSD doco help?
* http://jdebp.info/Proposals/freebsd-boot-manual-page.html
I wrote that for someone who observed that man boot was xyr goto, and it didn't exist on non-x86 architectures in FreeBSD.
I also wrote it up in a different way. The boot(8) manual for non-x86 was written to be an overview and pointers to other manual pages. The FGA is a slightly different approach.
http://jdebp.info/FGA/freebsd-boot-process.html
There are similar ideas on #NetBSD , although the names of things aren't the same. The FGA touches on the multiple stages thing. The reality of course is tha most of that goes away with #EFI.