[TAS] Add documentation for two-level scheduling TAS with examples of JobSet

**What would you like to be cleaned**:
Test manually how two-level scheduling for JobSet works and add documentation. Particularly cover 3 scenarios, when Jobset uses:
- just `podset` TAS annotations
- just `podset-slice` TAS annotations
- both `podset` and `podset-slice` annotations

**Why is this needed**: