RESP3: Clarify push message type encoding

[414owen]

The spec states:

and the first element of the array is always a String item, representing the kind of push data the server is sending to the client

I think this needs clarifying. Can the first element be a simple string or a blob string? The pubsub docs suggest it can be a blobstring, but the examples given in the spec use simple strings (albeit with a bundled caveat that the redis implementation uses blobstrings).

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So, can the first element be a simple string?