Cleaned up events.rst

Was a mix-up between TestEventSubscriber and EventTest (e.g. the definition of TestEventSubscriber referenced TestEvent::preFoo, which did not exist). To clarify this I've renamed EventTest to TestEvent.

Tried to clarify the text in the Naming Convention section.

Added note that onClear is not a lifecycle callback.

Tried to clarify the definition of Lifecycle Callbacks.

Separated key/value descriptions into XML and YAML parts since the details are different

Added note in Implementing Event Listeners section that since 2.4 you do have access to EntityManager and UnitOfWork from lifecycle callbacks.

Added example about how to use the computed changeset to modify a primitive value in preUpdate section

Added naming convention example to Entity listeners class section

The other changes are typos and small fixes.