scripts/kernel-doc: simplify signature printing

Untangle some of the $is_macro logic and the nested conditionals.

This makes it easier to see where and how the signature is actually
printed.

No functional change.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215134828.1277109-5-vegard.nossum@oracle.com