Small tweaks to the Doxygen documentation of the SF module.

doc/Doxyfile.in

src/parallel_io.c

src/serial_io.c

src/single_io.c

src/star_formation.h

src/star_formation/EAGLE/star_formation.h

src/star_formation/EAGLE/star_formation_io.h

src/star_formation/EAGLE/star_formation_struct.h

src/star_formation/none/star_formation_io.h

src/star_formation_io.h

src/star_formation_struct.h