"Sidecar" missing from definitions

[Lestropie]

Permalink to current version (1.10.0): https://bids-specification.readthedocs.io/en/v1.10.0/common-principles.html#definitions

In looking through the specification preparing a wave of posts on the Inheritance Principle, it seems that not only is the term "sidcar" missing from the list of definitions, but also---just like "entity" in the past (#947)---different people may be using the same term to mean different things:

• Is any key-value metadata JSON file a "sidecar"?
• Is a "sidecar" specifically a key-value metadata JSON file that has exactly the same filename stem as a data file, differing only in file extension?

In the presence of the Inheritance Principle, these two definitions are not the same.

[effigies]

As far as the schema and validator are concerned, the "sidecar" is the collection of keys and values aggregated through the inheritance principle. So I would say a sidecar file is any JSON file that provides metadata for another file, as opposed to for the dataset as a whole (like dataset_description.json or genetic_info.json).