ktime: fix some scripts/kernel-doc warnings

When building the htmldocs (in verbose mode), scripts/kernel-doc reports the
following type of warnings:
Warning(include/linux/ktime.h:75): No description found for return value of
'ktime_set'

Fix them by using a "Return:" section to describe the return values.
(Also apply some minor reformatting along the way.)

Signed-off-by: Yacine Belkadi <yacine.belkadi.1@gmail.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>