The English datasheet and user manual for the D1 are available at https://linux-sunxi.org/D1
However, some of the peripheral hardware (such as the video unit/frame buffer) is not documented and the documentation for the C906 CPU core itself is only available in Chinese.
Not really, the C906 source code (close-source) is written in Vperl (a custom internal language) which is translated into verilog by an internal tool. The OpenC906 (open-source) is only the generated verilog code from a light version of the C906 Vperl code, with some hand-made modifications/fixes.
The Vperl code and the tools to process it are not made public. However, the code generator leaves the Vperl code in verilog commentaries which gives an idea of the original code, and shows that big chunk of code have been removed/patched by hand.
I've mostly read code from the OpenC910, but I can tell you for sure that the C906 design and the OpenC906 design does not only differ only in some "updates and bugfixes". It's another code generated from a more advanced design which is not open-source
Edit: Not telling that the OpenC906 doesn't give you an idea of how the C906 is made, just that the C906 may contain a lot of differences with the OpenC906 that you don't know.