add documentation to generate_combos
add documentation to generate_combos

pierrelb/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py