Add some documentation to debug.h.
Add some documentation to debug.h.

hsorby/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml