[Docs] SSO OpenID oAuth2 - token_endpoint_auth_method

### Describe the Request

I am unsure which auth_method directus supports? One of your identity providers asked us if we would like to use "client_secret_post" or "client_secret_basic". As I cannot find any information about that in the docs, i would like to suggest a small information.

I only found an issue about supporting all openid-client configurations: https://github.com/directus/directus/discussions/20299

Therefore I assume directus only support

### Maintenance Strategy

Only when SSO is changing.