DOC: Improve visibility of RecaptchaField testing behavior

[OmeirP]

I'd like to update the Recaptcha validator class-level docstring to specify that the validator will always pass when in testing mode.

Although it does state it in the docs, since this isn't typically the case for other validators, it's easy to assume something else isn't behaving properly so having that information in a more obvious place would reduce confusion.

[OmeirP]

Realised it would make more sense to say it in the field's docstring over the validator's since it is unnecessary to add the validator to the field, since it's included by default.