Link to the List user's media admin API from media Admin API docs (#9571)

Earlier [I was convinced](https://github.com/matrix-org/synapse/issues/9565) that we didn't have an Admin API for listing media uploaded by a user. Foolishly I was looking under the Media Admin API documentation, instead of the User Admin API documentation.

I thought it'd be helpful to link to the latter so others don't hit the same dead end :)