Synopsis:
nmcli con modify -<property>.<setting> <value>

'value' can be empty ("") to remove the whole property value value (in this
case the behaviour is the same as without '-').
Or the 'value' is an index of the item to remove, or an option name (for a few
properties that have option names, like bond.options or ethernet.s390-options).

$ nmcli con mod myeth ipv4.dns "10.0.0.55 10.0.0.66 8.8.8.8 8.8.4.4"
---> ipv4.dns: 10.0.0.55, 10.0.0.66, 8.8.8.8, 8.8.4.4
$ nmcli con mod myeth -ipv4.dns 1
---> ipv4.dns: 10.0.0.55, 8.8.8.8, 8.8.4.4

---> bond.options: mode=balance-rr
$ nmcli con mod bond0 +bond.options "mii=500, downdelay=800"
---> bond.options: downdelay=800,miimon=500,mode=balance-rr
$ nmcli con mod bond0 -bond.options downdelay
---> bond.options: miimon=500,mode=balance-rr