Update annotations in Game.java

Methods that can return null need to be annotated as @Nullable in line with the conventions found here: https://github.com/SpongePowered/SpongeAPI/blob/master/CONTRIBUTING.md
Update EventManager JavaDocs spelling

Simple spelling change. Nothing too big.
Update VoxelEvent.java JavaDocs

Changed the wording in the JavaDocs so it flows much better.
Update ChunkEvent.java JavaDcos

Made the JavaDocs much clearer.
Update GameState.java JavaDocs

It's important for people to know what these GameStates actually mean, they need to be documented just like in spongepowered.api.event.Result.java. I don't know if these are exactly what the original author of this enum intended them to mean, but hopefully these can act as a point of discussion and base reference.
Update Block.java JavaDocs

We shouldn't call a Block a voxel when we have a voxel already defined under the World class. Calling it a Voxel rather than a block will just confuse people.
Merge pull request #2 from Nateb1121/patch-1

Add JavaDocs updates to Master branch.