scripts: kernel_doc: better handle show warnings logic

The logic with inhibits warnings for definitions that is not
output is incomplete: it doesn't cover the cases where
OUTPUT_INTERNAL and OUTPUT_EXPORTED are used.

As the most common case is OUTPUT_ALL, place it first,
in order to optimize a litte bit the check logic.

Fixes: 2defb272

 ("scripts: kernel-doc: apply filtering rules to warnings")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-and-Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>