Improve documentation for the `Mat4` projection functions

[kovaxis]

Like I mentioned in #569, this PR adds more detailed documentation to the Mat4::perspective_* and Mat4::orthographic_* family of functions, as well as adding Mat4::perspective_rh_vk and Mat4::orthographic_rh_vk.

I initially wrote a very detailed doc explaining what world-view coordinates get mapped to what normalized-device-coordinates, but they look very out-of-place and are probably too much. Instead, I added a small subtext among the lines of Useful to map the standard right-handed coordinate system into what OpenGL expects..

I haven't implemented the *_rh_vk functions or their tests yet. I would like to receive confirmation that this PR is ok before doing so.