Documentation: igb: Add a section about CBS

Add some pointers to the definition of the CBS algorithm, and some
notes about the limits of its implementation in the i210 family of
controllers.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>