Documentation: Content corrections for reCAPTCHA docs

What does this MR do?

Specifies that reCAPTCHA v2 tokens must be used. I originally tried using v3 tokens, not knowing the difference, and this resulted in an error. This clarifies that.

Also some small wording updates, and additional clarity on where in the Admin console the reCAPTCHA settings can be found.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards