Outdated docs on background diffusivity/viscosity

[whitleyv]

Some of the docs still include the background viscosity in the description of Smagorinsky-Lilly. Maybe you're purposefully keeping the "Physics" different than the "Model," but if it's a typo, these are the two places it's the old version:

Oceananigans.jl/docs/src/physics/turbulence_closures.md

Lines 75 to 81 in 738d172

	The eddy viscosity is given by
	```math
	\begin{align}
	\nu_e = \left ( C \Delta_f \right )^2 \sqrt{ \Sigma^2 } \, \varsigma(N^2 / \Sigma^2) + \nu \, ,
	\label{eq:smagorinsky-viscosity}
	\end{align}
	```

Oceananigans.jl/docs/src/physics/turbulence_closures.md

Lines 104 to 109 in 738d172

	where the eddy diffusivity ``\kappa_e`` is
	```math
	\kappa_e = \frac{\nu_e - \nu}{Pr} + \kappa \, ,
	```
	where ``\kappa`` is a constant isotropic background diffusivity.
	Both ``Pr`` and ``\kappa`` may be set independently for each tracer.

This description in the model setup is also a bit misleading, or, at least, unclear.

Oceananigans.jl/docs/src/model_setup/turbulent_diffusivity_closures_and_les_models.md

Lines 71 to 72 in 738d172

	although they may be specified. By default, the background viscosity and diffusivity are assumed to
	be the molecular values for seawater. For more details see [`SmagorinskyLilly`](@ref).

[glwagner]

Maybe you're purposefully keeping the "Physics" different than the "Model,"

I don't think so!

Previously we made the argument that the background viscosity is not part of the "eddy viscosity", conceptually. And of course as you point out that's not how its implemented currently either. So I agree this should be fixed.

This description in the model setup is also a bit misleading, or, at least, unclear.

Totally, this is wrong, there is no background viscosity of diffusivity when using SmagorinskyLilly either settable or by default.