docs: add Doxygen comments

[pascalecu]

I was frustrated with the lack of documentation in parts of this library, so I decided to at least attempt to document every metod, field and class/struct I could.

The reason this PR is so big is because I went through everything (and added about 2300 lines in the process :P).

Please look through it (and especially at the end at opt, I am a bit unsure there). @MikeMirzayanov