doc method
Gets a DocumentReference for the specified Firestore path.
Implementation
DocumentReference<Map<String, dynamic>> 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 _JsonDocumentReference(this, _delegate.doc(documentPath));
}