scripts/kernel-doc: check that non-void fcts describe their return value

If a function has a return value, but its kernel-doc comment doesn't contain a
"Return" section, then emit the following warning:

   Warning(file.h:129): No description found for return value of 'fct'

Note: This check emits a lot of warnings at the moment, because many functions
don't have a 'Return' doc section. So until the number of warnings goes
sufficiently down, the check is only performed in verbose mode.

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