Fixes in schema checker/doc to sync with spreadsheet + VisibilityXds => CorrelatedDataXds

[FedeMPouzols]

Resolves #257, with further fixes to sync the schema/doc/conversion/spreadsheet.
(half or more of this issue's branch was already included in #256)

Following on #256, this also renamed VisibilityXds => CorrelatedDataXds in the schema and doc .rst files.

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Note well:

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