docs: add an explanation about prefetch on hover to the link.mdx

[suu3]

Hello, I noticed the following code in link.tsx.

//client/link.tsx
...
onMouseEnter(e) {
    ...
    if (
        (!prefetchEnabled || process.env.NODE_ENV === 'development') &&
        isAppRouter
    ) {
        return
    }

    prefetch(
          ...
    )
},

It seems that when prefetch={false} is set, prefetch still occurs on hover for page routers.
However, there is no explanation in the documentation, which caused misleading.
I added the explanation inside PageOnly component, could you please review it?

[samcx]

Thank you for submitting a !

[ijjk]

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[samcx]

@suu3 No need to merge the latest canary!