minor javadoc fixes

Fixed most issues from the Ongoing Minor Issues List: #221 

Also, in EntityEvent the getEntity() javadoc specified the Entity returned as performing an action, but PlayerEvent does not specify the same Entity (In this case a Player) as performing an action, which may be confusing.

For every Optional, there should be something like "if available" in the javadocs.
Should it say "if available" in the method description or in the @ return ?