Improve ControlPersist documentation

[Alan Barrett]

On Fri, 13 Jun 2014, Loïc Le Loarer wrote:
> While testing the ControlPersist option (which is very useful 
> by the way, thank you), I find out that setting it to 0 has the 
> same behaviour as setting it to yes, while I would have expected 
> to exit as soon as the last client exits.

I think it should exit immediately, so the current behaviour of 
ControlPersist=0 waiting forever should be considered a bug to be 
fixed in code, not a feature to be added to documentation.

> Of course, it would also be a good idea to change the behaviour 
> and exits as soon as the last client exits. I tried to change 
> the code in this way, but there is an issue in that the control 
> background process doesn't even wait for the first client to 
> connect...

If fixing it properly is too difficult, then waiting for a very 
short time, like 1 second, might be a reasonable compromise.

--apb (Alan Barrett)