doc(model): add missing tbd about typing directive

@mizu/model/mod.html

@@ -23,6 +23,9 @@
       href="https://developer.mozilla.org/docs/Web/HTML/Element/textarea"
     ><code>&lt;textarea&gt;</code></a>. For other elements, use the <a href="#bind"><code class="hljs-keyword">:<span class="hljs-meta">attribute</span></code></a> directive.
   </mizu-restriction>
+  <mizu-tbd #note>
+    The <code>.nullish</code>, <code>.boolean</code>, <code>.number</code>, and <code>.string</code> modifiers are currently implemented as boolean modifiers, but future versions may change this behavior to offer more parsing features.
+  </mizu-tbd>
   <mizu-note #note>
     You can use the shorthand syntax <code><span class="hljs-keyword">::</span>="model"</code> instead of <code><span class="hljs-keyword">::<span class="hljs-meta">value</span></span>="model"</code>.
   </mizu-note>