improve the documentation of gs_design_ahr and gs_power_ahr

[LittleBeannie]

Update the @param, @return and delete the @section in gs_design_ahr and gs_power_ahr.

[LittleBeannie]

Hi @jdblischak, I updated the documentation of gs_design_ahr and gs_power_ahr. Please review and check if everything is clearer now.

[jdblischak]

This is great! The difference between gs_design_ahr() and gs_power_ahr() is much clearer now, and I appreciate the new details about the return object.

My only request is that the key distinction in the Description ("Calculate sample size and bounds given the power" for gs_design_X() and "Calculate the power given the sample size" for gs_power_X()) be propagated to all the gs_design_X() and gs_power_X() functions. My initial confusion arose when comparing gs_design_wlr() and gs_power_wlr(), and the Description for these functions in unchanged. You can address this request in this PR or in a a follow-up one.