improve docs

Author: vadzim-vys

libnavigation-core/src/main/java/com/mapbox/navigation/core/MapboxNavigation.kt

@@ -856,8 +856,7 @@ class MapboxNavigation @VisibleForTesting internal constructor(
     /***
      * Sets routes to preview.
      * Triggers an update in [RoutesPreviewObserver] and changes [MapboxNavigation.getRoutesPreview].
-     * Routes preview requires async processing, so new routes won't be immediately available in [RoutesPreviewObserver] and [MapboxNavigation.getRoutesPreview].
-     * Subscribe for updates using [MapboxNavigation.registerRoutesPreviewObserver] to receive new routes preview when processing will be completed.
+     * Preview state is updated asynchronously as it requires the SDK to process routes and compute alternative metadata. Subscribe for updates using [MapboxNavigation.registerRoutesPreviewObserver] to receive new routes preview state when the processing will be completed.
      *
      * If [routes] isn't empty, the route with [primaryRouteIndex] is considered as primary, the others as alternatives.
      * To cleanup routes preview state pass an empty list as [routes].