media: videobuf2-core: improve kernel-doc markups

Now that nested structs are supported, change the
documentation to use it. While here, add cross-references
where pertinent and use monotonic fonts where pertinent,
using the right markup tags.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>