scripts/kernel-doc: Do not track section counter across processed files

The section counter tracks how many sections of kernel-doc were added.
The only real use of the counter value is to check if anything was
actually supposed to be output and give a warning is nothing is
available.

The current logic of remembering the initial value and then resetting
the value then when processing each file means that if a file has the
same number of sections as the previously processed one, a warning is
incorrectly given.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241008082905.4005524-1-wenst@chromium.org