nicira-ext: Correct and improve nx_flow_update_abbrev comment.

Notifications always precede a barrier that follows a given flow_mod,
not a barrier that precedes a flow_mod as the text here previously claimed.

Also, the text about abbreviated notifications is equally applicable
to normal, unabbreviated notifications, so say that explicitly.

Reported-by: Jean Tourrilhes <jt@hpl.hp.com>
Acked-by: Jean Tourrilhes <jt@hpl.hp.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>