Document propagate_task_deploy setting

This is a bug about Fuel 9.1 documentation.

Some significant changes in task graph processing were done in Nailgun during Fuel 9.1 development. In particular, tasks with version < 2.0.0 aren't allowed to execute by default. If one still wants to use "old" tasks (with version less than 2.0.0) they should enable a special setting called "propagate_task_deploy" on Fuel UI. The problem is that the vast majority of fuel-plugins are using tasks with version < 2.0.0 and all of a sudden they stopped working with 9.1 while working okay with 9.0. Fuel documentation doesn't contain a word about those new features and how to deal with propagate_task_deploy.

I would like to see documented the next things:

- What is the reason for plugins developers to use tasks v2.0.0
- How 'propagate_task_deploy' setting works (who, when, and why should get it enabled)