Add docs section addressing SpectralCube transition

[rosteen]

Now that we're transitioning functionality from spectral-cube into specutils (e.g. #753, #755, #758), there should be a section in the docs that explains these functions and any differences between the way they were implemented in spectral-cube (e.g. the fact that they're manipulation functions rather than class methods, different upper bound handling in spectral_slab).