improve comments

src/RefElemData.jl

@@ -207,21 +207,32 @@ Polynomial{T}() where {T} = Polynomial(T())
     MultidimensionalQuadrature
 
 A type parameter for `Polynomial` indicating that the quadrature 
-has no specific structure. 
+has no specific structure. Example usage: 
+```julia
+# these are both equivalent
+approximation_type = Polynomial{MultidimensionalQuadrature}() 
+approximation_type = Polynomial(MultidimensionalQuadrature())
+```
 """
 struct MultidimensionalQuadrature end
 
 """
     TensorProductQuadrature{T}
 
 A type parameter to `Polynomial` indicating that the quadrature has a tensor 
-product structure. 
+product structure. Example usage: 
+```julia
+# these are both equivalent
+approximation_type = Polynomial{TensorProductQuadrature}(gauss_quad(0, 0, 1)) 
+approximation_type = Polynomial(TensorProductQuadrature(gauss_quad(0, 0, 1)))
+```
 """
 struct TensorProductQuadrature{T}
     quad_rule_1D::T  # 1D quadrature nodes and weights (rq, wq)
 end
 
 TensorProductQuadrature(args...) = TensorProductQuadrature(args)
+Polynomial{TensorProductQuadrature}(args) = Polynomial(TensorProductQuadrature(args))
 
 """
     TensorProductGaussCollocation

src/RefElemData_polynomial.jl

@@ -1,4 +1,6 @@
-# The following functions determine what default quadrature type to use 
+# The following functions determine what default quadrature type to use for each element.
+# For tensor product elements, we default to TensorProductQuadrature. 
+# For simplices, wedges, and pyramids, we default to MultidimensionalQuadrature
 
 # simplices and pyramids default to multidimensional quadrature
 RefElemData(elem::Union{Line, Tri, Tet, Wedge, Pyr}, 

src/explicit_timestep_utils.jl

@@ -6,8 +6,10 @@ Runge Kutta method. Coefficients evolve the residual, solution, and local time,
 
 # Example
 ```julia
-res = rk4a[i]*res + dt*rhs # i = RK stage
-@. u += rk4b[i]*res
+for i in eachindex(rk4a, rk4b)
+    @. res = rk4a[i] * res + dt * rhs # i = RK stage
+    @. u  += rk4b[i] * res
+end
 ```
 """
 function ck45()