scripts/kernel-doc: separate out function signature

Format the entire function signature and place it in a separate variable;
this both makes it easier to understand what these lines of code are doing
and will allow us to simplify the code further in the following patch.

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/20240215134828.1277109-4-vegard.nossum@oracle.com