Add Doxygen comments to all public functions

[ChrisThrasher]

A first step towards improved documentation should come in the form of writing Doxygen headers above all public functions. This will get readers more information on what each function does.

Eventually I'd like to create a static documentation website based on these. I've already done this for another repo of mine here so I know it can be done entirely via free GitHub services.

Let's target 2.6.x for this so that both v2 and v3 can benefit from it. Only one function in v2 is not present in v3 so this will not be difficult to do.