add docstring for batch_input in the transit.sign_data

hvac/api/secrets_engines/transit.py

@@ -806,6 +806,13 @@ def sign_data(
         :type salt_length: str | unicode
         :param mount_point: The "path" the method/backend was mounted on.
         :type mount_point: str | unicode
+        :param batch_input: Specifies a list of items for processing.
+            When this parameter is set, if the parameter 'hash_input' is also set, it will be ignored.
+            Responses are returned in the 'batch_results' array component of the 'data' element of the response.
+            Any batch output will preserve the order of the batch input.
+            If the input data value of an item is invalid, the corresponding item in the 'batch_results'
+            will have the key 'error' with a value describing the error.
+        :type batch_input: List[Dict[str, str]]
         :return: The JSON response of the request.
         :rtype: dict
         """
@@ -861,6 +868,9 @@ def sign_data(
                     allowed_types=transit_constants.ALLOWED_SALT_LENGTHS.pattern,
                 )
             )
+
+        hash_input = None if batch_input is not None else hash_input
+
         params = {
             "input": hash_input,
         }