Add proper documentation on background/critical steps

[k-dovgan]

The mechanism of background/critical nested steps is now quite complicated. So we really need to explain whole "what stops what, what waits for what" thing somewhere in documentation, with examples and references.

For example, the idea of critical substep only preventing some steps from launching, or background steps no longer considered substeps, or "wait for background" finishing only those steps already running, but not all the background steps in the config.