add docs for generalized power relative permeability

...eneralizedPower/c_RelativePermeabilityGeneralizedPower.md

...operty/RelativePermeabilityGeneralizedPower/t_exponent.md

...rmeabilityGeneralizedPower/t_min_relative_permeability.md

...erty/RelativePermeabilityGeneralizedPower/t_multiplier.md

...PermeabilityGeneralizedPower/t_residual_gas_saturation.md

...meabilityGeneralizedPower/t_residual_liquid_saturation.md

.../c_RelativePermeabilityGeneralizedPowerNonwettingPhase.md

...PermeabilityGeneralizedPowerNonwettingPhase/t_exponent.md

...alizedPowerNonwettingPhase/t_min_relative_permeability.md

...rmeabilityGeneralizedPowerNonwettingPhase/t_multiplier.md

...eralizedPowerNonwettingPhase/t_residual_gas_saturation.md

...lizedPowerNonwettingPhase/t_residual_liquid_saturation.md