Add comment about splashScreen.setKeepOnScreenCondition

app/src/main/java/com/google/samples/apps/nowinandroid/MainActivity.kt

@@ -76,7 +76,9 @@ class MainActivity : ComponentActivity() {
             }
         }
 
-        // Keep the splash screen on-screen until the user data is loaded
+        // Keep the splash screen on-screen until the UI state is loaded. This condition is
+        // evaluated each time the app needs to be redrawn so it should be fast to avoid blocking
+        // the UI.
         splashScreen.setKeepOnScreenCondition {
             when (uiState) {
                 Loading -> true