selftests/harness: Clean up kern-doc for fixtures

The FIXTURE*() macro kern-doc examples had the wrong names for the C code
examples associated with them. Fix those and clarify that FIXTURE_DATA()
usage should be avoided.

Cc: Shuah Khan <shuah@kernel.org>
Fixes: 74bc7c97 ("kselftest: add fixture variants")
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>