In order to calculate the idleSlope parameter of CBS correctly, users
must take into account the entire packet size, including the overhead
from all layers.
Add some more details to the man page to clarify that, giving one
simple example and pointing users to the correct 802.1Q section for
further clarifications if needed.
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>