qapi: improve TransactionAction doc

TransactionAction is a flat union, document 'type' versions
exhaustively, and sort the members.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170113144135.5150-6-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
