formats/zeiss: document zstd compression

[bgilbert]

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[bgilbert]

@iewchen, could you give this a quick check?

[openslide-bot]

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[iewchen]

Thanks! Looks good.

That zstd1 header can have size other than 3 bytes, however in practice it is always 3 regardless whether HiLo is used. I'd say this version is correct according to our best knowledge. We can improve it if Zeiss releases more ZSTD details.

[bgilbert]

Ah yeah, good point. I've updated the wording to equivocate a bit more on the header length.