docs(write-microflows): note @caption is ignored on loops, use @annotation

Document the MDL042 gotcha in the user-facing skill: loops have no caption in
Mendix, so @caption on a loop is dropped — use @annotation to attach a note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ako

.claude/skills/mendix/write-microflows.md

@@ -438,8 +438,20 @@ begin
     set $ActiveCount = $ActiveCount + 1;
   end if;
 end loop;
+
+-- Label a loop with a note (loops have no caption in Mendix)
+@annotation 'Process each product'
+loop $Product in $ProductList
+begin
+  set $count = $count + 1;
+end loop;
 ```
 
+> **`@caption` does nothing on a loop.** Mendix for-loops have no caption
+> property, so `@caption` on a `loop` is silently dropped (`mxcli check` flags
+> it as **MDL042**). To label a loop, use `@annotation 'text'` — it attaches a
+> note, exactly like drawing one onto the loop in Studio Pro.
+
 **Note**:
 - Loop variable (`$Product`) is scoped to the loop body
 - The loop variable type is **automatically derived** from the list type (e.g., `list of Test.Product` → `Test.Product`)