PortAssigner javadoc should explain restriction on ServerFactory

[sleberknight]

The class-level javadoc in PortAssigner should be amended to note that:

• the ServerFactory must be provided
• that ServerFactory must be an instance of DefaultServerFactory

We can also add the reason why the builder accepts a ServerFactory instead of DefaultServerFactory, which is that Dropwizard's Configuration#getServerFactory returns ServerFactory, so this allows passing that instance through to PortAssigner without a type check and cast. It is therefore a convenience, since the type check occurs during construction of PortAssigner.