More elaboration on establishment and termination of TCP

First, I want to thank for the awesome 'what-happens-when'.

However, when I see the chapter of `Opening of a socket`, I think it may be better to elaborate more and be more precise on the establishment and termination of TCP protocol, because this part is a really amazing design of the whole protocol. And practically speaking, many interviewers tend to ask this and follow up some questions about it.

Here is a small example of modifications can be done to this part:
> The other sides ACKs the FIN packet and sends its own FIN

I think it is better to emphasize that these two actions are normally not bound together.

So even if we are trying to keep the text simple here, I think an extra link or two to something more informative may be helpful (especially the design of state machine in termination and the three-way handshake).

Thank you!