Doc clarification on order of startup by priority

[ssteinerx]

Since the example in the documentation starts all the pingXs in the order that matches the order they're read from the file system, it wasn't clear to me that there was another mechanism at work and I was puzzled.

Naming the individual config files in something other than alphabetical order would help as would pointing out that the order of execution is priority dependent.

I'm jotting this down mostly as a note to myself to put together a pull request with this doc change if you agree it would be helpful and that they'll get merged.

Thanks!