While the Apple ][ documentation was so complete that it included Woz's annotated 6502 assembler source listings (I still have my copies in storage), the provided documentation was less extensive for the Atari 400/800 and Vic-20/C-64...
However, that's not really the point.
Those old-school bits of tin had no abstraction layers, so for coders to make any use of them beyond the basic (pardon the pun), they needed to address the hardware directly. I still remember having to load assembler routines on the Apple in order to toggle the speaker port sufficiently often to get actual musical tones (as I am equally old-school).
This really is not the case today - you only *need* that level of *hardware* documentation if you're going to write low-level OS drivers, and that's really beyond the scope of the project in question. The documentation of the abstraction platforms that sit on top of this Pi hardware are extensively... extensive, and are more than sufficient for most educational situations.
In addition, part of the learning process is discovering and understanding the limitations of the platform you're using, and deciding if you want to progress your learning further...