Resolving misc issues and ambiguities in labels section

[virginiascarlett]

Following up on my last PR. Previously, we added a lot more explanation to the 'labels' section of the spec. Now I am not just editing the language, but making some substantive, though minor, changes. Here's a quick summary of what I changed:

• Added for clarity: "Each label image MUST be stored within a "labels" group."
• One MUST --> MAY: "the datasets key MUST have the same number of entries (scale levels) as the original" --> "the datasets key MAY have the same number of elements (scale levels) as the original"
• One SHOULD --> MUST + clearer language: "In addition to the multiscales key, the JSON object in this image-level .zattrs file SHOULD contain another key, image-label, whose value is also a JSON object." --> "The label image .zattrs file MUST also contain the image-label key, whose value is a JSON object."
• Changed for clarity: "This array contains one JSON object for each unique custom label" --> "This array SHOULD contain one JSON object for each unique custom label."

Thanks!

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[imagesc-bot]

This pull request has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/save-a-single-labels-dataset-into-an-ome-zarr/93505/17

[jni]

Added for clarity: "Each label image MUST be stored within a "labels" group."

I would like to point to this imagesc discussion where people are quite unanimous that this is a bad idea — can we remove this line?