move note into docstring

Author: ianhi

examples/widgets/span_selector.py

@@ -8,14 +8,12 @@
 
 Here, an x-range can be selected on the upper axis; a detailed view of the
 selected range is then plotted on the lower axis.
-"""
 
-#############################################################################
-# .. note::
-#
-#    If the SpanSelector object is garbage collected you will lose the
-#    interactivity.  You must keep a hard reference to it to prevent this.
-#
+.. note::
+
+    If the SpanSelector object is garbage collected you will lose the
+    interactivity.  You must keep a hard reference to it to prevent this.
+"""
 
 import numpy as np
 import matplotlib.pyplot as plt