docs: Add TypeScript note for Symbol.dispose in Signals models

Users calling model[Symbol.dispose]() directly need ESNext.Disposable
in their tsconfig lib. The library typings themselves no longer require
it (fixed in preactjs/signals#911), but the symbol
still needs to be typed when referenced in app code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JoviDeCroock

content/en/guide/v10/signals.md

@@ -591,6 +591,8 @@ todoList.addTodo('Walk the dog');
 todoList[Symbol.dispose]();
 ```
 
+> **TypeScript:** If you call `model[Symbol.dispose]()` directly, add `"ESNext.Disposable"` (or `"ESNext"`) to the `lib` array in your `tsconfig.json` so that `Symbol.dispose` is typed.
+
 ### Recommended Patterns
 
 #### Explicit ReadonlySignal Pattern
@@ -808,6 +810,8 @@ console.log(counter.doubled.value); // 12
 counter[Symbol.dispose]();
 ```
 
+> **TypeScript:** Calling `model[Symbol.dispose]()` requires `"ESNext.Disposable"` (or `"ESNext"`) in your `tsconfig.json` `lib` array.
+
 ### action(fn)
 
 The `action(fn)` function wraps a function to run in a batched and untracked context. This is useful when you need to create standalone actions outside of a model: