Document the preprocess_esm function.

features/esm.py

@@ -35,6 +35,19 @@ def get_esm_data(usernames: Collection) -> pd.DataFrame:
 
 
 def preprocess_esm(df_esm: pd.DataFrame) -> pd.DataFrame:
+    """
+    Convert timestamps into human-readable datetimes and expand the JSON column into several Pandas DF columns.
+
+    Parameters
+    ----------
+    df_esm: pd.DataFrame
+        A dataframe of esm data.
+
+    Returns
+    -------
+    df_esm_preprocessed: pd.DataFrame
+        A dataframe with added columns: datetime in Ljubljana timezone and all fields from ESM_JSON column.
+    """
     df_esm["datetime_lj"] = df_esm["double_esm_user_answer_timestamp"].apply(
         lambda x: datetime.datetime.fromtimestamp(x / 1000.0, tz=TZ_LJ)
     )