Update Cache.xml (#2720)

I think it'd be more clear if it was explained that using the Add method to add to an already existing key fails "silently" rather than throwing an exception, or something along those lines. I was expecting "fails" was going throw an exception just like how dictionaries work. What do you guys think?

Author: prasanthlouis

xml/System.Web.Caching/Cache.xml

@@ -114,7 +114,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- Calls to this method will fail if an item with the same `key` parameter is already stored in the `Cache`. To overwrite an existing `Cache` item using the same `key` parameter, use the <xref:System.Web.Caching.Cache.Insert%2A> method.  
+ Calls to this method will fail silently if an item with the same `key` parameter is already stored in the `Cache`. To overwrite an existing `Cache` item using the same `key` parameter, use the <xref:System.Web.Caching.Cache.Insert%2A> method.  
   
  You cannot set both the `absoluteExpiration` and `slidingExpiration` parameters. If you intend the cache item to expire at a specific time, you set the `absoluteExpiration` parameter to the specific time, and the `slidingExpiration` parameter to <xref:System.Web.Caching.Cache.NoSlidingExpiration>.  
   
@@ -816,4 +816,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>