Code clean-up on EventSeries and EventInstance entity classes

Problem/Motivation

Let's tighten up the event entity classes to make DX a little nicer.

Proposed resolution

• Remove boilerplate comments.
• Some functions are using {@inheritdoc} when they don't actually have a parent or interface to inherit from. Either add those to the EventInterface or document the method.
• Some methods are overriding methods inherited from a base class that do functionally the same thing; let's get rid of those redundant implementations.
• Set return types on functions that don't have it.
• Remove redundant "$this instanceof" checks for interfaces that we already know are in the class hierarchy.
• Add an EventSeries::getSeriesEnd method (to go with the EventSeries::getSeriesStart method we already have).

API changes

Add EventSeries::getSeriesEnd method.