docs: add __CATEGORIES__ documentation (#98)

Co-authored-by: Erik Bjäreholt <erik.bjareholt@gmail.com>

src/examples/querying-data.rst

@@ -24,6 +24,22 @@ For an incomplete API reference of the transform functions, see the API referenc
 In a query you start by getting events from a bucket and assign that collection of events to a variable, then there are multiple transform functions which you can use to for example filter, limit, sort, and merge events from a bucket.
 After that you assign what you want to receive from the request to the RETURN variable.
 
+Magic Variables:
+
+    There is a magic variable `__CATEGORIES__` you can use in the web UI's Query Explorer to include your configured categories in your query.
+
+    Here's an example of using this variable to find all events categorized as "Work"
+
+    .. code-block:: python
+
+        events = flood(query_bucket("aw-watcher-window_"));
+        not_afk = flood(query_bucket("aw-watcher-afk_"));
+        not_afk = filter_keyvals(not_afk, "status", ["not-afk"]);
+        events = filter_period_intersect(events, not_afk);
+        events = categorize(events, __CATEGORIES__);
+        events = filter_keyvals(events, "$category", [["Work"]]);
+        RETURN = events;
+
 Minimal example:
     Minimal query which only gets events from a bucket and returns it: