config docs: document on kytos.conf what "threadpool" value on api_concurrency_limit means

[viniarck]

This is for documenting on kytos.conf.template what "threadpool" value on api_concurrency_limit means, and when an integer value should be used: https://github.com/kytos-ng/kytos/blob/master/kytos/templates/kytos.conf.template#L163

Turns out neither in the changelog nor in the template config it's explained. Ops team has asked for it.

@Ktmi since you contributed to it, can you enhance the docs?