[Docu] remove unneeded parameter documentation.

Documentation/ProjectFile/prj/processes/process/RichardsComponentTransport/porous_medium/i_porous_medium.md

-The id of the porous medium as an integer between 0 and the number of material
-groups minus one. There must be a corresponding data array 'MaterialIDs' in the
-mesh file.

Documentation/ProjectFile/prj/processes/process/RichardsComponentTransport/porous_medium/porous_medium/a_id.md

-It specifies the material ID of a porous medium. The material IDs of elements
-must be given in the mesh's cell data array "MaterialIDs".

Documentation/ProjectFile/prj/processes/process/RichardsComponentTransport/porous_medium/porous_medium/i_porous_medium.md

-A tag for the properties of a porous medium with material ID.
\ No newline at end of file

Documentation/ProjectFile/prj/processes/process/RichardsComponentTransport/porous_medium/porous_medium/t_capillary_pressure.md

-Capillary pressure model.