Add clarification to SuppressMessageAttribute (#4055)

I see `// SuppressMessage attributes do not apply to compiler diagnostics.` in Roslyn, but I think it is useful to make that clear in the documentation.

Tagging @BillWagner

Author: jcouv

xml/System.Diagnostics.CodeAnalysis/SuppressMessageAttribute.xml

@@ -44,7 +44,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Suppresses reporting of a specific static analysis tool rule violation, allowing multiple suppressions on a single code artifact.</summary>
+    <summary>Suppresses reporting of a specific static analysis tool rule violation, allowing multiple suppressions on a single code artifact. Does not apply to compiler diagnostics.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[