docs: document defeq and type inference related functions

[rish987]

Hi, I've been adding some documentation while trying to understand this implementation better. Regarding the FIXMEs, I've left them in for now just because I was curious if you have any thoughts on them (I think I will also do a bit of experimentation regarding them), but I can remove them before merging. I plan to add some docs for the type-inference functions next.