docs: kernel-doc.rst: improve structs chapter

There is a mess on this chapter: it suggests that even
enums and unions should be documented with "struct". That's
not the way it should be ;-)

Fix it and move it to happen earlier.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>