kernel-doc: new P directive for DOC: sections

The !P directive includes the contents of a DOC: section
given by title, e.g.
!Pfilename Title of the section

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>