bonding: document undocumented active_slave sysfs entry.

v2, based on Jay's review.

I kept the 'link must be up' part, because this is enforced in the code.

Signed-off-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>