Docs transport property calls

docs/src/transport_properties.md

@@ -1,5 +1,19 @@
 # Transport Properties
 
+After constructing a model, transport properties can be calculated (if respective parameters are available).
+The call for all this is always of the form:
+
+`transport_property(model, p, T, x=[1.]; phase=:unkwown)`
+
+where
+- `model` is a `AbstractTransportPropertyModel`,
+- `p` is the pressure (`[p] = Pa`),
+- `T` is the temperature (`[T] = K`),
+- `x` is the mole fraction (`[x] = mol mol⁻¹`), and
+- `phase` is the desired phase (`liquid` or `gas`) used in the volume solver.
+
+Internally, the density is always calculated first and then the function `EntropyScaling.ϱT_transport_property(model, ϱ, T, x)` is called (with `[ϱ] = mol m⁻³`).
+
 ```@docs
 EntropyScaling.viscosity
 EntropyScaling.thermal_conductivity