Recaptcha docs should be updated with threshold and fail message

https://bolt.github.io/forms/captcha.html

could use an update with the info from here:

https://github.com/bolt/forms/blob/8149c9082422d9208c5ba57045599d3788ab7f9a/docs/captcha.md

I found this because pages with forms were throwing exceptions about non existing indexes

[github-actions]

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