[Docs] Document Touchable hover callbacks (#4399)

## Description

Documents `onHoverIn`/`onHoverOut` on the `Touchable` page, together
with the behaviour that isn't obvious from the signature: how the two
pair up, what `disabled` does to an open hover, which platforms report
the crossings during a press, tvOS driving hover from focus, and hover
state not surviving an unmount or a detach.

## Test plan

`yarn start` in `packages/docs-gesture-handler`.