Deleted unwanted methods, added Javadoc to the ones we keep

May want to check the javadoc there... It's probably less than ideal. Maybe we should pull in the docs team on that?
Also, review the comments on the `Cache` interface... am unsure about some methods there
`.clear()` is better, doesn't imply anything more than clearing state (i.e. no remove events being fired, no spec'ed atomicity...