datapath: Improve <linux/openvswitch.h> comments.

datapath: Improve <linux/openvswitch.h> comments.

Generally we've used the comments to the right of attribute enums to
explain the types of the arguments and the ones above them to explain their
meaning.  This is a reasonable separation since it ensures that the type
of the argument is obvious, which in my opinion is important.

This updates a few comments to match this pattern.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>