Update documentation

pkgs/unified_analytics/test/log_handler_test.dart

@@ -123,6 +123,10 @@ void main() {
   test('Valid json but invalid keys', () {
     // The second line here is missing the "events" top level
     // key which should cause an error for that record only
+    //
+    // Important to note that this won't actually cause a FormatException
+    // like the other malformed records, instead the LogItem.fromRecord
+    // constructor will return null if all the keys are not available
     final contents = '''
 {"client_id":"fe4a035b-bba8-4d4b-a651-ea213e9b8a2c","events":[{"name":"lint_usage_count","params":{"count":1,"name":"prefer_final_fields"}}],"user_properties":{"session_id":{"value":1695147041117},"flutter_channel":{"value":null},"host":{"value":"macOS"},"flutter_version":{"value":"3.14.0-14.0.pre.303"},"dart_version":{"value":"3.2.0-140.0.dev"},"analytics_pkg_version":{"value":"3.1.0"},"tool":{"value":"vscode-plugins"},"local_time":{"value":"2023-09-19 14:44:11.528153 -0400"}}}
 {"client_id":"fe4a035b-bba8-4d4b-a651-ea213e9b8a2c","WRONG_EVENT_KEY":[{"name":"lint_usage_count","params":{"count":1,"name":"prefer_for_elements_to_map_fromIterable"}}],"user_properties":{"session_id":{"value":1695147041117},"flutter_channel":{"value":null},"host":{"value":"macOS"},"flutter_version":{"value":"3.14.0-14.0.pre.303"},"dart_version":{"value":"3.2.0-140.0.dev"},"analytics_pkg_version":{"value":"3.1.0"},"tool":{"value":"vscode-plugins"},"local_time":{"value":"2023-09-19 14:44:11.565549 -0400"}}}