Clarify display_html docs in case of no HTML repr

If there is no HTML representation for the object, nothing is shown.

See also  #9172.

IPython/core/display.py

@@ -185,6 +185,9 b' def display_pretty(*objs, **kwargs):'
 
 def display_html(*objs, **kwargs):
     """Display the HTML representation of an object.
+    
+    Note: If raw=False and the object does not have a HTML
+    representation, no HTML will be shown.
 
     Parameters
     ----------