Message search should be available for guests

**Suggestion**

The message search endpoint, [`POST /_matrix/client/v3/search`](https://spec.matrix.org/v1.5/client-server-api/#post_matrixclientv3search), should be available for guests to use.

For reference, these are the current guest access endpoints: https://spec.matrix.org/v1.5/client-server-api/#client-behaviour-14

Read the POST /_matrix/client/v3/search section in the Matrix Client-Server API and compare it with the linked guest access endpoints and client behaviour reference. Update the specification so the requested guest availability is reflected consistently, then verify the endpoint and guest-access sections agree.

Written by the indexing model from the issue text.