fix(docs): add node(Warning|Critical)WindowHours to node-mixin

[Duologic]

Adding a config option:

// Some of the alerts use predict_linear() to fire alerts ahead of time to
// prevent unrecoverable situations (eg. no more disk space). However, the
// node may have automatic processes (cronjobs) in place to prevent that
// within a certain time window, this may not align with the default time
// window of these alerts. This can cause these alerts to start flapping.
// By reducing the time window, the system gets more time to
// resolve this before problems occur.

[discordianfish]

Ugh sorry for missing this! Not sure why this slipped though.