landlock: Add documentation for UDP support

landlock: Add documentation for UDP support

Add example of UDP usage, without detailing the two access right.
Slightly change the example used in code blocks: build a ruleset for a
DNS client, so that it uses both TCP and UDP.

Test coverage for security/landlock is 91.3% of 2245 lines according to
LLVM 22.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://patch.msgid.link/20260611162107.49278-7-matthieu@buffet.re
[mic: Fix doc formatting, update audit doc, add test coverage]
Signed-off-by: Mickaël Salaün <mic@digikod.net>