Before this change, enums that have one member were formatted as, e.g.:
"one of xyzzy, , or "
This changes them to be formatted as:
"must be xyzzy"
which makes much more sense.
(An enum with one member may make some sense if you are trying to leave
the possibility for future expansion.)
Signed-off-by: Ben Pfaff <blp@nicira.com>