doc method
Returns a reference to a single document by id.
Use idField to override the collection identifier field for this
document reference only.
Implementation
RealtimeDocumentReference<T> doc(String id, {String? idField}) {
return RealtimeDocumentReference<T>(
client: _client,
collection: _name,
id: id,
idField: idField?.trim() ?? _idField,
fromJson: _fromJson,
);
}