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.

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[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)