Apple does not need to provide documentation.
Apple uses Intel processors which have their own design specifications.
The Intel Sandy Bridge processors in the MacBook Pro support up to 16GB RAM. That's an Intel specification not an Apple one.
For the MacBook Pro 2011, with 2 memory slots, that's 2x8GB.
When Apple designed the MacBook Pro 2011, 8GB modules were not readily available, so, the max RAM was 2x4GB.
Btw, why would a vendor sell memory that isn't going to work with your MacBook Pro?
Aren't users going to complain and return the modules?
Don't users have to buy their RAM from a vendor that can supply it?