Make clear how to remove an unmanaged layer from a map

src/ol/layer/layer.js

@@ -149,8 +149,9 @@ ol.layer.Layer.prototype.handleSourcePropertyChange_ = function() {
 /**
  * Sets the layer to be rendered on a map. The map will not manage this layer in
  * its layers collection, and the layer will be rendered on top. This is useful
- * for temporary layers. To add the layer to a map and have it managed by the
- * map, use {@link ol.Map#addLayer} instead.
+ * for temporary layers. To remove an unmanaged layer from the map, use
+ * `#setMap(null)`. To add the layer to a map and have it managed by the map,
+ * use {@link ol.Map#addLayer} instead.
  * @param {ol.Map} map Map.
  * @api
  */