add note on bad pattern in docstring

daft/expressions/expressions.py

@@ -795,6 +795,7 @@ def concat(self, other: str) -> Expression:
 
     def extract(self, pattern: str | Expression, index: int = 0) -> Expression:
         """Extracts the regex match group from each string in a string column. If index is 0, the entire match is returned, otherwise the specified group is returned.
+        If the pattern does not match or the group does not exist, a null value is returned.
 
         Example:
             >>> df = daft.from_pydict({"x": ["foo", "bar", "baz"]})