Issue #3622733: Say that the remote client rebinds the client contract only, not the engine

Scopes the symmetry statement in docs/distributed.md to OrchestraClientInterface, which is the only contract orchestra_client rebinds, and says which submodules need a local engine and why. Also carries the action task variable-target warning down from the node settings form into docs/actions.md, so someone designing a model from the documentation sees it too.