[Laupetin]

Description of Problem

The Transition and TransitionGroup built-in components expose a persisted prop that is not included in the documentation, but in the shipped typescript types.
Therefore various code editors or IDEs suggest this prop but neither the shipped types nor the documentation explain what it does.

Proposed Solution

This PR adds documentation for the persisted prop of the Transition and TransitionGroup built-in components.
The text is taken from the corresponding comment in code (line breaks were adjusted to be readable and not overflow in rendered documentation):

https://github.com/vuejs/core/blob/0ff253bfba3ee8d3d65448bd44ad890fd01b0ecd/packages/runtime-core/src/components/BaseTransition.ts#L37-L42

Additional Information

I am not sure whether this prop is actually intended to be part of the public API.
If not, this PR can just be closed again.
However it may be a problem then that the typescript types for the props make editors suggest parts of an internal API then?

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