describe birth death process better

[MUCDK]

https://moscot.readthedocs.io/en/latest/notebooks/examples/problems/800_score_genes_for_marginals.html

explain the parameters

[selmanozleyen]

hi, I noticed in my PR #696 I specified these parameters. Also changed the implementation a little bit. Just as a heads up, I'd recommend modifiying the docstrings here as this might be merged