[doxygen] Adapt documentation in 2p sequential

dumux/porousmediumflow/2p/sequential/celldata.hh

dumux/porousmediumflow/2p/sequential/celldataadaptive.hh

...diumflow/2p/sequential/diffusion/cellcentered/pressure.hh

.../2p/sequential/diffusion/cellcentered/pressureadaptive.hh

...p/sequential/diffusion/cellcentered/pressureproperties.hh

...tial/diffusion/cellcentered/pressurepropertiesadaptive.hh

.../2p/sequential/diffusion/cellcentered/pressurevelocity.hh

...diumflow/2p/sequential/diffusion/cellcentered/velocity.hh

.../2p/sequential/diffusion/cellcentered/velocityadaptive.hh

...smediumflow/2p/sequential/diffusion/mimetic/croperator.hh

...low/2p/sequential/diffusion/mimetic/croperatoradaptive.hh

...iumflow/2p/sequential/diffusion/mimetic/localstiffness.hh

...rousmediumflow/2p/sequential/diffusion/mimetic/mimetic.hh

...umflow/2p/sequential/diffusion/mimetic/mimeticadaptive.hh

...ousmediumflow/2p/sequential/diffusion/mimetic/operator.hh

...mflow/2p/sequential/diffusion/mimetic/operatoradaptive.hh

...ousmediumflow/2p/sequential/diffusion/mimetic/pressure.hh

...mflow/2p/sequential/diffusion/mimetic/pressureadaptive.hh

...low/2p/sequential/diffusion/mimetic/pressureproperties.hh

...equential/diffusion/mimetic/pressurepropertiesadaptive.hh