Add clarity for media endpoint discovery

[MaybeThisIsRu]

In 3.6.1, the spec says the server MUST include media-endpoint in a config request:

To advertise that the Micropub endpoint supports a Media Endpoint, the server MUST include a key called media-endpoint with a value of the full URL of the Media Endpoint in the Micropub configuration request. Clients MUST NOT assume the Media Endpoint is on the same domain as the Micropub endpoint.

In 3.7.1, the spec says the server SHOULD include media-endpoint in a config request:

The server SHOULD include the following information in the configuration response.

• The list of syndication endpoints supported, in a property called syndicate-to (see Syndication Targets for details on the structure of the response)
• The Media Endpoint if supported, in a property called media-endpoint

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In either section, the response is only to include a media-endpoint if it is supported. However, the language used makes it hard to decide whether a server must, or must not. It would be great if this could be spelled out clearly and uniformly.