[Train] Update Lightning RayDDPStrategy docstring

[woshiyyya]

Why are these changes needed?

Ray Train will start a distributed group with the arguments specified in TorchConfig before the training function. if the distributed process group has already been fired up, Lightning will ignore some arguments(backend, timeout).

We need to point the users to specify these arguments in TorchConfig instead of RayDDPStrategy.

Related issue number

Closes #36315

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[matthewdeng]

We can also parse the args and print a warning if these values are set.