Inconsistent MPI marker docs/enums

[Helveg]

The MPIMarkerEnum class lists 4 markers: mpi, mpi_skip, mpi_xfail and mpi_break; the MPI_MARKERS constant right below lists only 3 of them (misses mpi) and the marker docs don't mention mpi_break.

https://github.com/aragilar/pytest-mpi/blob/9ad369af744a47cedf5025245f051e793703c748/src/pytest_mpi/init.py#L19-L39

[aragilar]

The missing documentation is accidental, that should be fixed.

The MPI_MARKERS constant is correct—it's mapping the intent of the marker when being run under MPI to the appropriate pytest mark (as the mpi marker need do nothing when run under MPI, rather the inverse is true).