Write narrative documentation to explain what Inbound/Outbound mean

[Lukasa]

Given that channelRead is the most prominent ChannelInboundHandler method, and that write is the most prominent ChannelOutboundHandler method, it's easy for users to assume that inbound means reads, and outbound means writes. This will cause user confusion when they discover that channelWritabilityChanged is an Inbound method, and that read is an Outbound one!

We should write some narrative docs for these two protocols to make it clear that this is not actually how they work: instead, that inbound/outbound is about causality. Specifically, Inbound events are things the socket tells the pipeline, while Outbound events are things the pipeline tells the socket. From that construction it becomes clear why read is Outbound (the pipeline tells the socket to do a read), and why channelWritabilityChanged is Inbound (the socket tells the pipeline whether it's writable or not).