Include description of the index in application docs

docs/user_guide/preprocessing/application.ipynb

@@ -15,11 +15,10 @@
     "\n",
     "Application data refers to the information about which apps are open at a certain time. These data can reveal important information about people's circadian rhythm, social patterns, and activity. Application data is an event data; this means it cannot be sampled at a regular frequency. Instead, we just have information about the events that occured. \n",
     "\n",
-    "An application data dataframe  contains the following columns (column names can be different, but in that case they must be provided as parameters):\n",
+    "An application data dataframe contains a list of application events. Each row describes a single event and is indexed by a time stamp. The dataframe contains the following columns (column names can be different, but in that case they must be provided as parameters):\n",
     "- `user`: Subject ID.\n",
     "- `device`: Device ID.\n",
     "- `app_column`: Contains the name of an application used.\n",
-    "- ``\n",
     "\n",
     "There are two main issues with application data (1) missing data detection, and (2) privacy concerns.\n",
     "\n",