Document withDefaults in https://vuejs.org/guide/typescript/composition-api.html

[segevfiner]

What problem does this feature solve?

The useful withDefaults SFC macro is not documented in https://vuejs.org/guide/typescript/composition-api.html only the reactivity transform based syntax, this might make it seem that that's the only way to use TypeScript based declarations with defaults. It should be documented there as well.

What does the proposed API look like?

Updated documentation.