Document all background-* property parsers and make them behave correctly.
Document all background-* property parsers and make them behave correctly.

svn path=/trunk/libcss/; revision=7857

prepare/netsurf_libcss,servo/libcss,prepare/netsurf_libcss,servo/libcss,servo/libcss,prepare/netsurf_libcss,zhuyadong/libcss,zhuyadong/libcss,zhuyadong/libcss