Clarify that format is required if formatted_body is specified

[Johennes]

The current spec of format and formatted_body dates back to matrix-org/matrix-spec-proposals#1397 which documented the status quo at the time. It includes an explicit note that formatted_body is required if format is specified but doesn't state whether or not formatted_body can occur without format.

I assume the intention was for formatted_body not to be allowed without format as clients would have struggled to infer the format from just formatted_body if further formats had ever been introduced. As further evidence, ruma doesn't allow one of the two to be optional without the other and Element Web will simply fall back to body if formatted_body is specified without format.

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