"bind" specifically binds the lifetime of the activation (NMActiveConnection).
In combination with "persist=volatile", the lifetime of the NMSettingsConnection
is indirectly bound to the NMActiveConnection. But still these concepts make sense
independently.
In the future, it may make sense to also bind the lifetime of the NMSettingsConnection
to the D-Bus client. Hence, rename the option to allow for the distinction.

Also, belatedly fix libnm comment about "bind" only working with
"persist" "volatile".

Fixes: eb883e34