Document Default decision_policy and forecast_policy

[nhandler]

We should document in https://paasta.readthedocs.io/en/latest/autoscaling.html#how-to-use-autoscaling that the default decision_policy is proportional and the default forecast_policy is current

[thebostik]

Another small change to this page when switching examples around... bdouglas realized that pid is still documented as a decision_policy and used in examples, but the code is gone since #1654.