putDoc method
Writes an already encoded document (including its header) as a nested doc.
Implementation
void putDoc(int fieldId, Bytes doc) {
_beginEntry(fieldId, VxsWire.lengthDelimited);
_writeVarint(doc.length);
_ensure(doc.length);
_data.setRange(_pos, _pos + doc.length, doc.asUint8List());
_pos += doc.length;
}