Man pages matching name mpi_info_get_nthkey in section 3:

MPI_Info_get_nthkey.3.en	debian 10 / lam-mpidoc
MPI_Info_get_nthkey.3.en	debian 10 / mpich-doc
MPI_Info_get_nthkey.3.en	debian 11 / lam-mpidoc
MPI_Info_get_nthkey.3.en	debian 12 / lam-mpidoc
MPI_Info_get_nthkey.3.en	debian 12 / mpich-doc