docs: improve async guidance and examples

examples/NestedLayouts/src/main.cpp

@@ -196,11 +196,16 @@ void renderToSerial() {
   while (!TemplateRenderer::isComplete(ctx) && !TemplateRenderer::hasError(ctx)) {
     size_t written = TemplateRenderer::renderNextChunk(ctx, buffer, sizeof(buffer));
     if (!written) {
+      Serial.println(F("\nRendering stalled before completion."));
       break;
     }
     Serial.write(buffer, written);
   }
-  Serial.println();
+  if (TemplateRenderer::hasError(ctx)) {
+    Serial.println(F("\nRendering failed."));
+  } else if (TemplateRenderer::isComplete(ctx)) {
+    Serial.println();
+  }
 }
 
 }  // namespace
@@ -218,4 +223,3 @@ void setup() {
 void loop() {
   // Nothing to do in loop for this example.
 }
-