[Doc] Added parameter documentation

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_MECHANICS/t_initialize_porosity_from_medium_property.md

+This optional parameter indicates whether the porosity and transport porosity
+should be initialized from the medium properties `porosity` and
+`transport_porosity`, respectively.
+
+This initialization is desired in almost all cases with one notable exception:
+simulation restart when a non-constant porosity model is used,
+e.g. with \ref ogs_file_param__prj__media__medium__properties__property__PorosityFromMassBalance "PorosityFromMassBalance"
+or \ref ogs_file_param__prj__media__medium__properties__property__TransportPorosityFromMassBalance "TransportPorosityFromMassBalance".
+
+In such a restart case simply set this parameter to `false`.