workqueue: fix some scripts/kernel-doc warnings

When building the htmldocs (in verbose mode), scripts/kernel-doc reports the
following type of warnings:

Warning(kernel/workqueue.c:653): No description found for return value of
'get_work_pool'

Fix them by:
- Using "Return:" sections to introduce descriptions of return values
- Adding some missing descriptions
Signed-off-by: Yacine Belkadi <yacine.belkadi.1@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>