scripts: kernel-doc: Replace the usage function

Aim: unified POD, user more satisfied, script better structured

You can see the results with:

$ scripts/kernel-doc -help

Signed-off-by: Tomasz Warniełło <tomasz.warniello@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Disliked-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220218181628.1411551-10-tomasz.warniello@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>