Separate Docstring GradientVariablesEntropy (#2420)

* Separate Docstring `GradientVariablesEntropy`

* add links

Author: DanielDoehring

src/equations/compressible_navier_stokes.jl

@@ -51,18 +51,26 @@ struct Adiabatic{F}
 end
 
 """
-`GradientVariablesPrimitive` and `GradientVariablesEntropy` are gradient variable type parameters
-for `CompressibleNavierStokesDiffusion1D`. By default, the gradient variables are set to be
-`GradientVariablesPrimitive`. Specifying `GradientVariablesEntropy` instead uses the entropy variable
-formulation from
+`GradientVariablesPrimitive` is a gradient variable type parameter for the [`CompressibleNavierStokesDiffusion1D`](@ref), 
+[`CompressibleNavierStokesDiffusion2D`](@ref), and [`CompressibleNavierStokesDiffusion3D`](@ref).
+The other available gradient variable type parameter is [`GradientVariablesEntropy`](@ref).
+By default, the gradient variables are set to be `GradientVariablesPrimitive`.
+"""
+struct GradientVariablesPrimitive end
+
+"""
+`GradientVariablesEntropy` is a gradient variable type parameter for the [`CompressibleNavierStokesDiffusion1D`](@ref), 
+[`CompressibleNavierStokesDiffusion2D`](@ref), and [`CompressibleNavierStokesDiffusion3D`](@ref).
+The other available gradient variable type parameter is [`GradientVariablesPrimitive`](@ref).
+
+Specifying `GradientVariablesEntropy` uses the entropy variable formulation from
 - Hughes, Mallet, Franca (1986)
   A new finite element formulation for computational fluid dynamics: I. Symmetric forms of the
   compressible Euler and Navier-Stokes equations and the second law of thermodynamics.
   [https://doi.org/10.1016/0045-7825(86)90127-1](https://doi.org/10.1016/0045-7825(86)90127-1)
 
 Under `GradientVariablesEntropy`, the Navier-Stokes discretization is provably entropy stable.
 """
-struct GradientVariablesPrimitive end
 struct GradientVariablesEntropy end
 
 """

src/equations/compressible_navier_stokes_1d.jl

@@ -17,7 +17,8 @@ the [`CompressibleEulerEquations1D`](@ref).
 - `mu`: dynamic viscosity,
 - `Pr`: Prandtl number,
 - `gradient_variables`: which variables the gradients are taken with respect to.
-                        Defaults to `GradientVariablesPrimitive()`.
+                        Defaults to [`GradientVariablesPrimitive()`](@ref).
+                        For an entropy stable formulation, use [`GradientVariablesEntropy()`](@ref).
 
 Fluid properties such as the dynamic viscosity ``\mu`` can be provided in any consistent unit system, e.g.,
 [``\mu``] = kg m⁻¹ s⁻¹.

src/equations/compressible_navier_stokes_2d.jl

@@ -17,7 +17,8 @@ the [`CompressibleEulerEquations2D`](@ref).
 - `mu`: dynamic viscosity,
 - `Pr`: Prandtl number,
 - `gradient_variables`: which variables the gradients are taken with respect to.
-                        Defaults to `GradientVariablesPrimitive()`.
+                        Defaults to [`GradientVariablesPrimitive()`](@ref).
+                        For an entropy stable formulation, use [`GradientVariablesEntropy()`](@ref).
 
 Fluid properties such as the dynamic viscosity ``\mu`` can be provided in any consistent unit system, e.g.,
 [``\mu``] = kg m⁻¹ s⁻¹.

src/equations/compressible_navier_stokes_3d.jl

@@ -17,7 +17,8 @@ the [`CompressibleEulerEquations3D`](@ref).
 - `mu`: dynamic viscosity,
 - `Pr`: Prandtl number,
 - `gradient_variables`: which variables the gradients are taken with respect to.
-                        Defaults to `GradientVariablesPrimitive()`.
+                        Defaults to [`GradientVariablesPrimitive()`](@ref).
+                        For an entropy stable formulation, use [`GradientVariablesEntropy()`](@ref).
 
 Fluid properties such as the dynamic viscosity ``\mu`` can be provided in any consistent unit system, e.g.,
 [``\mu``] = kg m⁻¹ s⁻¹.