Expand documentation of variable propagation to include parent-child relationships with CompositeEnvs

[bpkroth]

As discussed in https://github.com/microsoft/MLOS/pull/960/files/393e0388b3f56e234277172094a28a5f76c8e9bd#r2006264282 we should also document (with runnable examples) how variable propagation works in the context of CompositeEnv parent child relationships.