SGFixedVector2 clamp + docs

This adds a clamped method to SGFixedVector2 that clamps its magnitude to the value specified. Fairly straightforward but I wanted to get a handle on actually writing some C++ on a relatively simple task.

I've also fleshed out more of the documentation for the SGFixedVector2 class. I'm happy to break this down into two merge requests if it's too much trouble to review both at once.