Use consistent return types for available_monitors() (#1207)

* [#1111] Use consistent return types for available_monitors()

Always use `impl Iterator<Item = MonitorHandle>` instead of
`AvailableMonitorsIter`. Fix an example that used the Debug
implementation of `AvailableMonitorsIter`.

* [#1111] Update changelog

* [#1111] Remove AvailableMonitorsIter type completely

* [#1111] Remove doc references to AvailableMonitorsIter

examples/monitor_list.rs

@@ -4,6 +4,6 @@ fn main() {
     let event_loop = EventLoop::new();
     let window = WindowBuilder::new().build(&event_loop).unwrap();
 
-    dbg!(window.available_monitors());
+    dbg!(window.available_monitors().collect::<Vec<_>>());
     dbg!(window.primary_monitor());
 }