man: document IPx_METHOD and method-change action

When the IP connection method is changed, the 'method-changed' action
is triggered. This can be used to act on connection method changes
such as conection sharing switched to manual. The actual value is
provided by 'IP4_METHOD' and 'IP6_METHOD' environment variables.
56 jobs for method-change-dispatch in 29 minutes and 56 seconds (queued for 6 seconds)