doc method
Gets a DocumentReference for the specified Firestore path.
Implementation
DocumentReference doc(String documentPath) {
assert(
documentPath.isNotEmpty, 'a document path must be a non-empty string');
assert(!documentPath.contains('//'),
'a collection path must not contain "//"');
assert(isValidDocumentPath(documentPath),
'a document path must point to a valid document.');
return DocumentReference._(this, _delegate.doc(documentPath));
}