Fix documentation for changes implemented in v1.0.0-rc2

Per the changelog (v1.0.0-rc2):

CollectionView / CompositeView

    **BREAKING: ** Changed the `item:added` event to `before:item:added` and `after:item:added`
reorder documentation for consistency

change the order callbacks are covered, so it matches the order in the section on collection events
improve consistency between callback and event documentation

* document onItemRemoved callback
* remove references to corresponding callback within the event section: the callbacks are documented above, and introduced in the section's first paragraph