Feature: scheduler: priority-fencing-delay defaults to 0 meaning disabled

This commit also documents the upcoming new behavior as discussed from:

https://github.com/ClusterLabs/pacemaker/pull/2012

Any static/random delays that are introduced by pcmk_delay_base/max
configured for the corresponding fencing resources will be added to this
delay. This delay should be significantly greater than, safely twice,
the maximum pcmk_delay_base/max. By default, priority fencing delay is
disabled.