docs(v-model): document factory-function defaults for objects/arrays

[pcoterecollective]

What

The defineModel() default section on the v-model page only documents a primitive default (default: 0). Because defineModel() declares a regular prop, its default is resolved through the same machinery as defineProps — so an object/array default that isn't a factory function is shared across every component instance, and mutating the model in one instance leaks into the others.

This adds a short note + example right after the existing default example, and links to Prop Validation where the factory-function rule is already stated. The page even uses factory syntax for modelModifiers, just never for the value default.

Repro

Vue SFC Playground

• Object-literal default (default: {}): both instances render the same object id — one shared reference.
• Factory default (default: () => ({})): each instance renders a different object id — a fresh object per instance.

[netlify]

✅ Deploy Preview for vuejs ready!

Name	Link
🔨 Latest commit	6410dc0
🔍 Latest deploy log	https://app.netlify.com/projects/vuejs/deploys/6a5f8d2fdd9f0a0008b2a86d
😎 Deploy Preview	https://deploy-preview-3434--vuejs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pcoterecollective]

@brc-dd — mind a quick look when you get a chance? The v-model page only documents a primitive defineModel() default (default: 0), so object/array defaults and the factory-function requirement aren't covered. Repro + playground link are in the description.

[brc-dd]

I'm fine with adding it, but generally speaking you should never mutate a prop or model. The recommended way is to create a model and do an assignment to replace the value.

[whysopaul]

Just a quick note: a pull request with a similar proposal has already been opened (#3410)

[bencodezen]

Suggested change

	When the default value is an object or array, return it from a factory function, just as with [prop defaults](/guide/components/props#prop-validation). Otherwise every component instance that falls back to the default shares the **same** reference, and mutating the model in one instance leaks into the others:
	When the default value is an object or array, it will be shared across all instances of the component because it shares the same reference. In other words, changing the model in one instance will affect the other instances.
	Similar to [prop defaults](/guide/components/props#prop-validation), if you want the component instances to have their own unique values, you need to return it from a factory function.