Remove event pooling and SyntheticEvent#persist from documents

[koba04]

As the blog mentioned, React has stopped event pooling, and the SyntheticEvent#persist mthod does nothing in v17, so I remove the description from the documentation.

https://reactjs.org/blog/2020/08/10/react-v17-rc.html#no-event-pooling

v17 is still in RC, so this PR might have to be merged after releasing v17 stable.

Does React have a plan to deprecate and remove the SyntheticEvent#persist method in the future??

[gaearon]

There's a link in _redirects that links to this section. I think we need to add a separate page (unlisted) for this warning now.

[koba04]

@gaearon
Thank you. I've added a page for the legacy event pooling and updated the redirect link.

[gaearon]

Technically it's still there so it should be in the API reference. Just needs to note it doesn't do anything.

[koba04]

@gaearon
Thank you. I've added the note as another note at 89a9c29.

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[vercel]

@koba04 is attempting to deploy a commit to the Facebook Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[koba04]

React 17 has been released, so this PR is ready for review.

[gaearon]

Can you please rebase and then also mirror changes in vercel.json?

[vercel]

Deployment failed with the following error:

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[koba04]

I've rebased and modified a redirect link in vercel.json