add more comments to chat example

[hmeine]

Motivation

As discussed with Programatik and Ferry on Discord, people have lots of
questions about the chat example, so some more code comments might be
helpful.

Solution

As a new axum user myself (target audience of this example), I just tried to
express what I learned from carefully reading this example and implementing
something similar with slightly more words than before. A particular focus was
on the spawned tasks (hopefully useful for readers not too familiar with async
rust yet).

[davidpdrsn]

Thanks!

A very minor nit but would you change the : at the end of the comments back to either . or just nothing? We don't use that convention anywhere else in axum.

[davidpdrsn]

Thanks!