"…but it came with superb documentation for every bit of hardware and software in it,…"

Very true, I still have my XT and AT manuals and use them as illustrations of how documentation should be done. Back then, good documentation included both operational and service manuals including maintenance and circuit diagrams (and as you say IBM even included BIOS source code).

IBM documentation up until at least that point had always been suburb across its product range. For example I have the original copy of this IBM typewriter manual even though I've no longer the machine because the manual is so very excellent:

https://archive.org/details/bitsavers_ibmtypewriricTypewrite...

Manufacturers considered their products and manuals proprietary and protected them with patents and copyrights but they were not secret as is so often the case nowadays. Right to Repair wasn't even a concept back then, had anyone suggested such a notion they'd be laughed at, as it was automatically assumed. What counts as documentation nowadays is an utter disgrace.

IBM was not alone, companies such as HP, Tektronix, Racal (and many others) also produced truly excellent documentation—especially so their maintenance and repair manuals. Equipment performance could often be judged on the quality of the manuals alone.