GEAR: Fix documentation

doc/Doxyfile.in

src/chemistry/GEAR/chemistry.h

src/cooling/grackle/cooling.c

src/cooling/grackle/cooling_io.h

src/cooling/grackle/cooling_struct.h

src/feedback/GEAR/feedback.c

src/feedback/GEAR/feedback.h

src/feedback/GEAR/hdf5_functions.h

src/feedback/GEAR/interpolation.h

src/feedback/GEAR/stellar_evolution.c

src/feedback/GEAR/supernovae_ii.c

src/star_formation/GEAR/star_formation.h

src/star_formation/GEAR/star_formation_io.h

src/star_formation/GEAR/star_formation_logger.h

src/stars/GEAR/stars.h