ofproto: Fix comments.

The comments on the "group" functions had been shamelessly copied without
significant update from the corresponding flow table functions.  This
commit fixes the errors.

This commit also removes an obsolete comment in ofopgroup_complete().

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>