Add example of how to rate limit per-connection

[realyze]

I just got bitten by rate-limiter thinking by default the rule applies to each connection separately. This example should help educate users that it is not in fact the case unless you add connectionId() { return true; }.

[StorytellerCZ]

Hi @realyze !
Thanks for the submission! Could you maybe add comment into the connectionId function that explains why that part is required or at least highlights that it is required so that people don't throw it out by accident?

Thanks!