telemetry: Update docstrings after TimelineData patch.

tools/telemetry/telemetry/core/timeline/model.py

Index: tools/telemetry/telemetry/core/timeline/model.py
diff --git a/tools/telemetry/telemetry/core/timeline/model.py b/tools/telemetry/telemetry/core/timeline/model.py
index 832e6f8090588ae7ca15e041cd86b300100700c3..86165be44b045f724c53dc73cd06fc9e57bfdf60 100644
--- a/tools/telemetry/telemetry/core/timeline/model.py
+++ b/tools/telemetry/telemetry/core/timeline/model.py
@@ -41,6 +41,12 @@ class MarkerOverlapError(Exception):
 
 class TimelineModel(object):
   def __init__(self, timeline_data=None, shift_world_to_zero=True):
+    """ Initializes a TimelineModel. timeline_data can be a single TimelineData
+    object, a list of TimelineData objects, or None. If timeline_data is not
+    None, all events from it will be imported into the model. The events will
+    be shifted such that the first event starts at time 0, if
+    shift_world_to_zero is True.
+    """
     self._bounds = bounds.Bounds()
     self._thread_time_bounds = {}
     self._processes = {}