doc method
Gets a DocumentReference for the specified Firestore path.
Implementation
DocumentReference<Map<String, dynamic>> doc(String documentPath) {
if (documentPath.isEmpty) {
throw ArgumentError('A document path must be a non-empty string.');
} else if (documentPath.contains('//')) {
throw ArgumentError('A document path must not contain "//".');
} else if (!isValidDocumentPath(documentPath)) {
throw ArgumentError('A document path must point to a valid document.');
}
return _JsonDocumentReference(this, _delegate.doc(documentPath));
}