Edited the in Doxygen documentation in preview.c.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Correction in Doxygen function description in preview.c.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Edited Doxygen documentation of courtyard shape in preview.h.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>