doc: document the enum and bitfield attributes

Introduce the enum and bitfield attributes, which allow you to refer to the enum
you are expecting in an argument, and specify which enums are to be thought of
as bitfields.

Changes since v3:
 - Fix typo ("description" -> "descriptive")

Signed-off-by: Auke Booij <auke@tulcod.com>
Reviewed-by: Nils Chr. Brause <nilschrbrause@googlemail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>