Improve Caching Docs

[aidanm3341]

The caching docs should include more information, especially around the effects of channel disposal on any cached messages.

e.g.
This is how caching works:

const broker = messagebroker<IChannels>();
broker.create('channel', { replayCacheSize: 2 }).publish('Jupiter');
broker.get('channel').subscribe(message => console.log(message));
// expect: Jupiter

If you were to dispose of that channel before subscribing, then it behaves like this:

const broker = messagebroker<IChannels>();
broker.create('channel', { replayCacheSize: 2 }).publish('Jupiter');
broker.dispose('channel');
broker.get('channel').subscribe(message => console.log(message));
// expect: nothing...