docs: Use fallthrough pseudo-keyword

Replace /* fall through */ comment with fallthrough, make
it align with original process/coding-style.rst

Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210515155142.2490-1-jj251510319013@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>