Document flattenned data encoding

[Totktonada]

Rules for the record and record* are the following:

1. record on uplevel (inside tuple / array / union): encode with wrapping array;
2. record inside another record or record*: encode fields as is, w/o wrapping.
3. record* on uplevel: encode as nil or with wrapping array.
4. record* inside another record or record*: encode as nil or with wrapping array.

Others are pretty straightforward (and context unaware), but need to be explicitly stated too.