Add explanation about assumptions for start method

WordPress/src/main/java/org/wordpress/android/ui/posts/EditPostActivity.java

@@ -2669,6 +2669,10 @@ private void onEditorFinalTouchesBeforeShowing() {
 
     @Override
     public void onEditorFragmentContentReady(ArrayList<Object> unsupportedBlocksList) {
+        // Note that this method is also used to track startup performance
+        // It assumes this is being called when the editor has finished loading
+        // If you need to refactor this, please ensure that the startup_time_ms property
+        // is still reflecting the actual startup time of the editor
         mPostEditorAnalyticsSession.start(unsupportedBlocksList);
     }