CLEAR: Add note about debugging.

Author: mikewest

specs/clear-site-data/index.src.html

@@ -444,6 +444,11 @@ <h3 id="clear">
       [[#reload-contexts]] on <var>response</var>'s {{Response/url}}'s
       <a>origin</a>, with <var>subdomain state</var>.
 
+  Note: Especially given the cross-context implications, user agents are
+  are encouraged to give web developers some mechanism by which the clearing
+  operation can be debugged. This might take the form of a console message or
+  timeline entry indicating success.
+
   <h4 id="neuter-contexts">
     Neuter browsing contexts matching <var>origin</var> with
     <var>subdomain state</var>