scripts: kernel-doc: change default to ReST format

Right now, if kernel-doc is called without arguments, it
defaults to man pages. IMO, it makes more sense to
default to ReST, as this is the output that it is most
used nowadays, and it easier to check if everything got
parsed fine on an enriched text mode format.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>