How to check if guest registration is available, without creating a guest account?

**Link to problem area**:

https://spec.matrix.org/v1.17/client-server-api/#post_matrixclientv3register

**Issue**
What is wrong? How can we improve?

The spec only mentions `POST /_matrix/client/v3/register?kind=guest` may be called to register a guest account, but there is no way mentioned to check before making this call if guest registration is available.

If matrix clients simply call `POST /_matrix/client/v3/register?kind=guest` to check for 403/M_GUEST_ACCESS_FORBIDDEN, to validate if guest registration is available (e.g. in a situation to prompt the user if they want to create a full user account or a guest account up front), it will create a guest account if it IS available, which unnecessarily pollutes the server's database if the user isn't interested in creating a guest account.

Start with the POST /_matrix/client/v3/register?kind=guest section linked in the issue and review how guest registration availability is currently described. Define a protocol-safe way to check availability without creating an account, then update the relevant specification text and examples; done requires an agreed behavior that clients and servers can implement consistently.

Written by the indexing model from the issue text.