Add extra calrification for html legend (#11614)

Author: LeeLenaleee

docs/samples/legend/html.md

@@ -1,6 +1,7 @@
 # HTML Legend
 
-This example shows how to create a custom HTML legend using a plugin and connect it to the chart in lieu of the default on-canvas legend.
+This example shows how to create a custom HTML legend using a plugin and connect it to the chart in lieu of the default on-canvas legend.  
+For an html legend to work you need to place an empty div at your web page with the ID you provide in the options to bind to like so: `<div id="legend-container"></div>`.  
 
 <div id="legend-container"></div>