[Docu] document tag description.

Documentation/ProjectFile/prj/processes/process/ComponentTransport/t_chemically_induced_porosity_change.md

+\copydoc ProcessLib::ComponentTransport::ComponentTransportProcessData::chemically_induced_porosity_change

ProcessLib/ComponentTransport/ComponentTransportProcessData.h

@@ -38,6 +38,11 @@ struct ComponentTransportProcessData
     Eigen::VectorXd const specific_body_force;
     bool const has_gravity;
     bool const non_advective_form;
+    /// When this optional tag is on, the feedback of chemical reactions on the
+    /// porosity will be counted. The change of porosity equals to the summation
+    /// over the changes in the volume fractions of solid constituents. The
+    /// change of the volume fraction, in terms of a solid constituent, results
+    /// from chemical reactions.
     bool const chemically_induced_porosity_change;
     ChemistryLib::ChemicalSolverInterface* const chemical_solver_interface;