Add documenting comments to orderly.json schema

[david-mears-2]

To clarify the intent behind the properties/what terms refer to

[david-mears-2]

Unfortunately the 'windows-latest' R-CMD-check fails. I re-ran it and it failed again.

[plietar]

@richfitz how is this schema coordinated across repos (if at all)?

We have https://github.com/mrc-ide/outpack for the outpack schemas, but I can't find any equivalent for orderly.json.

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EDIT: Seems this is the one and only copy of that file that we have: https://github.com/search?q=org%3Amrc-ide+path%3Aorderly.json&type=code, so making this PR here seems good. At some point we should move it somewhere global, the outpack repo seems reasonable to me.

[plietar]

I couldn't reproduce the build failure on a Windows VM 😞

[richfitz]

The idea was that orderly2 is responsible for its schemas, it would hold them itself. It imports schemas from outpack (there's a script somewhere to pull them in)