doc: improve documentation of objects (5)

  ([`getState()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PropagatedCharge.html#ac14ea433405562d18a6113948707552f))

The following values for the carrier state are possible:

- `CarrierState::UNKNOWN`:

  The final state of the charge carrier is unknown, for example because it might not have been provided by the used

  propagation algorithm.

  propagation algorithm

- `CarrierState::MOTION`:

  The charge carrier was still in motion when the propagation routine finished, for example when the configured integration

  time was reached.

  time was reached

- `CarrierState::RECOMBINED`:

  The charge carrier has recombined with the silicon lattice at the given position.

  The charge carrier has recombined with the silicon lattice at the given position

- `CarrierState::TRAPPED`:

  The charge carrier has been trapped by a lattice defect at the given position.

  The charge carrier has been trapped by a lattice defect at the given position

- `CarrierState::HALTED`:

  The motion of the charge carrier has stopped, for example because it has reached an implant or the sensor surface.

  The motion of the charge carrier has stopped, for example because it has reached an implant or the sensor surface

For more details refer to the [code reference](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PropagatedCharge.html).