Update CAPTCHA documentation to mention turning off verify origin feature

changelog.d/10046.doc

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+Update CAPTCHA documentation to mention turning off the verify origin feature. Contributed by @aaronraimist.

docs/CAPTCHA_SETUP.md

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 # Overview
-Captcha can be enabled for this home server. This file explains how to do that.
-The captcha mechanism used is Google's ReCaptcha. This requires API keys from Google.
-
-## Getting keys
-
-Requires a site/secret key pair from:
-
-<https://developers.google.com/recaptcha/>
-
-Must be a reCAPTCHA v2 key using the "I'm not a robot" Checkbox option
-
-## Setting ReCaptcha Keys
-
-The keys are a config option on the home server config. If they are not
-visible, you can generate them via `--generate-config`. Set the following value:
-
+A captcha can be enabled on your homeserver to help prevent bots from registering
+accounts. Synapse currently uses Google's reCAPTCHA service which requires API keys
+from Google.
+
+## Getting API keys
+
+1. Create a new site at <https://www.google.com/recaptcha/admin/create>
+1. Set the label to anything you want
+1. Set the type to reCAPTCHA v2 using the "I'm not a robot" Checkbox option.
+This is the only type of captcha that works with Synapse.
+1. Add the public hostname for your server, as set in `public_baseurl`
+in `homeserver.yaml`, to the list of authorized domains. If you have not set
+`public_baseurl`, use `server_name`.
+1. Agree to the terms of service and submit.
+1. Copy your site key and secret key and add them to your `homeserver.yaml`
+configuration file
+    ```
     recaptcha_public_key: YOUR_SITE_KEY
     recaptcha_private_key: YOUR_SECRET_KEY
-
-In addition, you MUST enable captchas via:
-
+    ```
+1. Enable the CAPTCHA for new registrations
+    ```
     enable_registration_captcha: true
+    ```
+1. Go to the settings page for the CAPTCHA you just created
+1. Uncheck the "Verify the origin of reCAPTCHA solutions" checkbox so that the
+captcha can be displayed in any client. If you do not disable this option then you
+must specify the domains of every client that is allowed to display the CAPTCHA.
 
 ## Configuring IP used for auth
 
-The ReCaptcha API requires that the IP address of the user who solved the
-captcha is sent. If the client is connecting through a proxy or load balancer,
+The reCAPTCHA API requires that the IP address of the user who solved the
+CAPTCHA is sent. If the client is connecting through a proxy or load balancer,
 it may be required to use the `X-Forwarded-For` (XFF) header instead of the origin
 IP address. This can be configured using the `x_forwarded` directive in the
-listeners section of the homeserver.yaml configuration file.
+listeners section of the `homeserver.yaml` configuration file.