Add description to all exported function and their arguments

[wleoncio]

There are several function descriptions/arguments that contain a placeholder "TODO:" tag. Exported functions are functions that the package user can directly access, so they should be well documented.

[wleoncio]

This should wait until #13 is closed.

[wleoncio]

Hi @Theo-qua,

I saw your comment on #17 (comment). Thanks for working on this.

There seems to be a few missing parameters, however (see image below; I suppose the ones on the cpp file are easy to copy from the R counterpart):

Also, please run devtools::document() once you're done so the documentation files (*.Rd) can be updated.