Improved the clarity of the top-level API doc (#2984)

I clarified some of the language here so that it is easier for the beginner to understand. (It was a little bit awkward before)

docs/docs/top-level-api.md

@@ -1,6 +1,7 @@
 # Top Level API
 
-In order to make navigation API homogenous as much as possible, we provide setRoot function that receives layout of any kind.
+So as to make the navigation API as consistent and homogenous as possible, we begin with a single, unifying function -- setRoot -- that receives properties for any kind of layout, whether tabs or stacks.
+
 See [Layout types](docs/layout-types)
 
 
@@ -96,4 +97,4 @@ Navigation.registerScreen('example.AdvancedScreen', () => AdvancedScreen);
 
 In some cases you might need the id of the currently visible screen. This method returns the unique id of the currently visible screen:
 `const visibleScreenInstanceId = await Navigation.getCurrentlyVisibleScreenId()`
-In order to have any use of this method, you'd need to map instanceId to screens your self. -->
+In order to have any use of this method, you'd need to map instanceId to screens your self. -->