The search endpoint does not mention the list of allowed values for `room_events.keys`

**Link to problem area**: https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3search

**Issue**

The documentation for the `room_events.keys` parameter does not mention a restricted list of keys to use for search, implying (as per my own interpretation) that all keys in an event are searchable. However, since https://github.com/matrix-org/synapse/pull/307 (which predates the spec for this endpoint) Synapse has always restricted this list to `["content.body", "content.name", "content.topic"]`.

**Expected behaviour**

The spec should mention that the values for `room_events.keys` must be included in `["content.body", "content.name", "content.topic"]`.

Open the linked Matrix Client-Server API search section for POST /_matrix/client/v3/search and inspect the room_events.keys parameter description. Update the specification to state the allowed values listed in the issue, then verify that the parameter documentation no longer implies arbitrary event keys are searchable.

Written by the indexing model from the issue text.