Document required behaviour for stream-object-names skip function

[itowlson]

https://github.com/WebAssembly/wasi-blobstore/blob/005c4250d6d2c969dbc7d7294210c995b4fb86fd/wit/container.wit#L61C1-L64C81 says:

    // skip the next number of objects in the stream
    //
    // This function returns the number of objects skipped, and a boolean indicating if the end of the stream was reached.
    skip-stream-object-names: func(num: u64) -> result<tuple<u64, bool>, error>;

Is an implementation required to skip exactly num (unless it reaches end of stream) - that is, the caller can be sure that when the stream is next read (using read), it will be num objects further along? Or is it required that it skip no more than num, but it's allowed to skip fewer, and it's on the caller to keep polling it if it wants a specific number to be skipped? (I ask because of analogy to the read function, where I assume it is allowed to return fewer than len in each response - at least I hope it is because that's what I plan to do! - and to understand the intended use of the return value.)