Add Alpha comment to docs

docs/config.md

@@ -217,7 +217,7 @@ solver              displacementLaplacian;
 
 #### FF
 
-The fluid-fluid coupling module supports reading and writing `Pressure`, `Velocity`, `PressureGradient`, `VelocityGradient`, `FlowTemperature` and `FlowTemperatureGradient`.
+The fluid-fluid coupling module supports reading and writing `Pressure`, `Velocity`, `PressureGradient`, `VelocityGradient`, `FlowTemperature`, `FlowTemperatureGradient`, `Alpha` and `AlphaGradient`.
 
 Similarly to the CHT module, you need a `fixedValue` boundary condition of the respective primary field in order to read and apply values, and a `fixedGradient` boundary condition of the respective gradient field in order to read and apply gradients.
 
@@ -248,6 +248,17 @@ interface
 The FF module is still experimental and the boundary conditions presented here have not been rigorously tested.
 {% endexperimental %}
 
+`Alpha` refers to the phase variable used in e.g. the volume of fluid multiphase solver `interFoam`. When coupling multiphase flows the correct names for pressure and alpha need to be specified in the `preciceDict`.
+
+```c++
+// File preciceDict (example for interFoam)
+FF
+{
+  nameP       p_rgh;
+  nameAlpha   alpha.water;
+}
+```
+
 ### Volume coupling
 
 Besides surface coupling on the domain boundaries, the OpenFOAM adapter also supports coupling overlapping domains, which can be the complete domain, or regions of it. In contrast to surface coupling, though, reading volume data (source terms) requires a few additional configuration steps compared to writing data.