[Docu] add documentation for the optional tag.

Documentation/ProjectFile/prj/processes/process/StokesFlow/t_use_stokes_brinkman_form.md

+\copydoc ProcessLib::StokesFlow::StokesFlowProcessData::use_stokes_brinkman_form

ProcessLib/StokesFlow/StokesFlowProcessData.h

@@ -31,6 +31,8 @@ struct StokesFlowProcessData
     /// an external force that applies in the bulk of the fluid, like gravity.
     Eigen::VectorXd const specific_body_force;
 
+    /// When this optional field is flagged on, the Darcy term will be
+    /// additionally included in the original momentum balance equation.
     bool const use_stokes_brinkman_form;
 
     MeshLib::PropertyVector<double>* pressure_interpolated = nullptr;