[fbsync] Add explanation for injected fake data in dataset tests (#3576)

Reviewed By: fmassa

Differential Revision: D27127996

fbshipit-source-id: 64a9bbf801010555b0fa91ea72b8ff158c7260e1

Author: vincentqb

test/datasets_utils.py

@@ -266,6 +266,10 @@ def dataset_args(self, tmpdir: str, config: Dict[str, Any]) -> Sequence[Any]:
     def inject_fake_data(self, tmpdir: str, config: Dict[str, Any]) -> Union[int, Dict[str, Any]]:
         """Inject fake data for dataset into a temporary directory.
 
+        During the creation of the dataset the download and extract logic is disabled. Thus, the fake data injected
+        here needs to resemble the raw data, i.e. the state of the dataset directly after the files are downloaded and
+        potentially extracted.
+
         Args:
             tmpdir (str): Path to a temporary directory. For most cases this acts as root directory for the dataset
                 to be created and in turn also for the fake data injected here.