Corrections to asteroid code

Updated definitions of DiscoveryInfo.referenceLifetime and ScenarioDiscoverableObjects.spawnInterval.

Added best guesses for asteroid radii to UntrackedObjectClass. Clarified that ProceduralAsteroid.Generate serves as the main factory method for PAsteroid.
Merge branch 'master' of https://github.com/Anatid/XML-Documentation-for-the-KSP-API
Merge pull request #4 from Starstrider42/master

Asteroid Update