zm: Mention Value/OwnedValue conversion requirement for properties

Hi,

Currently neither https://docs.rs/zbus/latest/zbus/attr.dbus_interface.html nor https://dbus.pages.freedesktop.org/zbus/server.html mention that properties need Value/OwnedValue conversion while normal methods work without that. Since it is IMHO unexpected that property getter/setters handle their arguments differently than normal methods it should be documented.