docs: kdoc: clean up the create_parameter_list() "first arg" logic

docs: kdoc: clean up the create_parameter_list() "first arg" logic

The logic for finding the name of the first in a series of variable names
is somewhat convoluted and, in the use of .extend(), actively buggy.
Document what is happening and simplify the logic.

Acked-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250814154035.328769-4-corbet@lwn.net