Recaptcha docs should be updated with threshold and fail message

[Puddingboy]

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