Ambiguous usage "once observed"

[skynavga]

Document

https://vuejs.org/api/options-state.html#data

Improvement

Define or refer to definition of "observed" as used in

Once observed, you can no longer add reactive properties to the root data object.

N.B. The terms observe and observed are also used in the ECMAScript® 2023 Language Specification without any formal definition.

[skirtles-code]

This has been carried over from the Vue 2 docs. I agree it needs rewording. I'm also unclear what is meant by 'once observed'.