Add info about dimension ordering to docs

[oruebel]

2) Feature Request

Currently the ordering of dimensions described in the schema is in many cases not listed in the documentation. E.g., for ElectricalSeries.data we should add to the docval that the dimensions are num_time | num_channels. This would help users avoid errors with ordering of dimensions.

This issue was motivated by #960

This issue is in part also related to #626

Problem/Use Case

Help users avoid mistakes with ordering of dimensions by including the dimension names indicated in the spec also in the API docs.

Checklist

• [X] Have you ensured the feature or change was not already reported ?
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• [X] Have you included a minimal code snippet that reproduces the issue you are encountering?
• [X] Have you checked our Contributing document?