luci-app-firewall: suppress lengthy masq description in zone table

Ensure that the description of the masquerade option does not end up in
the grid section overview as it messes up the table layout.

Fixes: c54efde7

 ("luci-app-firewall: Add clarification to masquerading option")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>