Spec says that room directory is not authed. But in practice, synapse auths it by default.

**Link to problem area**:
https://spec.matrix.org/v1.13/client-server-api/#listing-rooms

**Issue**
Says that `GET /_matrix/client/v3/directory/list/room/{roomId}` doesn't require authentication

In practice, Synapse auths it by default:

https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#allow_public_rooms_without_auth

**Expected behaviour**

Spec should make it clear that servers can auth it (and the federation endpoints) and are encouraged to do so.

Read the room-directory listing section in the Matrix Client-Server API and compare its authentication wording with Synapse's allow_public_rooms_without_auth configuration documentation. Check the related federation endpoint language as well. Done means the specification clearly states that servers may require authentication and are encouraged to do so.

Written by the indexing model from the issue text.