Add 'Client' suffix to some 'World' methods to indicate that they only work on the client

[Octol1ttle]

• change: add 'Client' suffix to some 'World' methods to indicate that they only work on the client
  Self-explanatory.
• change(World#playSound): explain the behavior on client vs. server in more detail
  Explains that, on the client, the sound will play if it's the same as the client's player (source == this.client.player). On the server, every player except the source will get a sound packet.
• Add Javadocs to World#add(Important)ParticlesClient
  The Javadocs explain what an "important" particle is, what these methods do on the server (nothing), and reference the ServerWorld method used for spawning particles in case the developer is looking for it.