docs: kdoc: simplify the PROTO continuation logic
Remove the unneeded "cont" variable and tighten up the code slightly.
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250606163438.229916-2-corbet@lwn.net