It's not clear whether `/versions` should fail if auth is invalid

### Link to problem area:
https://spec.matrix.org/v1.10/client-server-api/#get_matrixclientversions
### Issue:
As of v1.10, the `/versions` endpoint has become the first to optionally accept authentication. However, it is not specified whether if authentication is provided and is invalid whether the server should return an error, or just perform like there was no authentication provided in the first place.

Read the Matrix Client-Server API v1.10 `/versions` section linked in the issue, along with its authentication rules. Determine and document whether invalid supplied authentication should produce an error or be treated as unauthenticated, then update the specification to state the chosen behavior clearly.

Written by the indexing model from the issue text.