[sqm-scripts/luci-app-sqm] Document how to disable shaping on a per direction basis

sqm-scripts for a long time interprets a "Down- or Upload speed" of zero as
an indication that the shaper should be disabled. Note that really shaping
an individual direction down  o zero will make the link effectively dead
for tcp (think reverse ACK traffic). Son instead of allowing the user to
configure something broken, 0 was "over-loaded" to denote no shaping
since several years, but that information has not been documented visibly
to the users. This commit aims at fixing that oversight.

Signed-off-by: Sebastian Moeller <moeller0@gmx.de>