Improve documentation for Camera3D's `current` member

* Added additional information to the camera documentation to explain
  how the 'current' attribute behaves when multiple cameras are in a
  scene.

doc/classes/Camera3D.xml

@@ -155,6 +155,7 @@
 		</member>
 		<member name="current" type="bool" setter="set_current" getter="is_current" default="false">
 			If [code]true[/code], the ancestor [Viewport] is currently using this camera.
+			If multiple cameras are in the scene, one will always be made current. For example, if two [Camera3D] nodes are present in the scene and only one is current, setting one camera's [member current] to [code]false[/code] will cause the other camera to be made current.
 		</member>
 		<member name="doppler_tracking" type="int" setter="set_doppler_tracking" getter="get_doppler_tracking" enum="Camera3D.DopplerTracking" default="0">
 			If not [constant DOPPLER_TRACKING_DISABLED], this camera will simulate the [url=https://en.wikipedia.org/wiki/Doppler_effect]Doppler effect[/url] for objects changed in particular [code]_process[/code] methods. See [enum DopplerTracking] for possible values.