Add description of what window.navigate() is for

Closes #70

Author: frivoal

index.bs

@@ -388,11 +388,26 @@ JavaScript API</h2>
 <h3 id=high-level-api>
 Triggering Navigation Programmatically</h3>
 
-Issue: This API is clearly useful for testing purposes.
-It may also be useful for authors,
-but the use cases are less strong.
-Maybe this should be moved to WebDriver and be a testing-only API.
-Maybe it is fine as is.
+The {{Window/navigate()}} method enables the author to trigger spatial navigation programatically,
+as if the user had done so manually
+(for instance, by pressing the arrow keys in a browser where that is the way to trigger spatial navigation).
+
+Note: As this triggers the same processing model as a manual navigation,
+all the same results should be expected:
+the same chain of events will be fired and
+the same element will be scrolled or focused.
+
+Note: Authors can use this to trigger spatial navigation
+based on a different UI mechanism than the one assigned by the UA,
+such as mapping to different keys,
+or triggering spatial navigation from a clickable on-screen directional pad.
+or in reaction to other events than UI ones.
+It could also be used when an author wants to interrupt navigation to do some asynchronous operation
+(e.g. load more content in an infinite scroller) then resume where they cancelled.
+
+Note: This API is also useful for testing purposes,
+as there is no other way to trigger spatial navigation
+that does not depend on vendor specific UI conventions.
 
 <pre class="idl">
 enum SpatialNavigationDirection {