DOC: compressor kwarg of create_array should be more thoroughly documented

[jni]

Describe the issue linked to the documentation

The Group.create_array docstring lists the following for "compressor":

compressor: dict[str, JSON] | None = None
    The compressor for the array.

This lacks the two pieces of information I was looking for:

• is it compressed by default?
• what are valid values for a compressor?

The second question is probably complicated, but it could be remedied by (1) linking out to the compressors documentation and/or (2) providing at least ONE example of a valid compressor.

Suggested fix for documentation

Default compressor behaviour should be listed, and a link to the compressors specification documentation should be provided.

[d-v-b]

also that type annotation is wrong, we should fix that too