Added functionality and descriptions to Missile projectile

Added a LockOnProbability, turned MissileCloseEnough from a hack into a feature, and added some descriptions.
It would probably makes sense here to clarify that setting this to 0 does *not* mean that the missile needs a direct hit ;)

Also, with this being configurable, I foresee problems where modders make missiles that are fast enough and have a `CloseEnough` value that is small enough and that then just fly right past their targets (similar to our bullets flying through walls problem). Adding something to that effect to the description would be worthwhile as well, I think.