[docs] Unique fields

[natew]

Notice there is uniqueItems documented, and that primary fields are assumed to be unique, but didn't see anything for just unique fields.

It does seem like it may work by searching code, but perhaps some tests + docs would help. I'm going to test to see if it works now as-is, and then could help with some docs.

[natew]

Ok did some verification, and it seems unique wasn't enforced for me. I changed the schema to have unique: true, it warned me it changed, cleared data, inserted a few things with the same property and it worked.

[pubkey]

We currently do not have unique fields.
uniqueItems is part of json-schema, but only works for elements of arrays.

[natew]

All good, doing a find on preInsert is good enough.