Add usage note to DisablePrivateReflectionAttribute (#4969)

Author: GrabYourPitchforks

xml/System.Runtime.CompilerServices/DisablePrivateReflectionAttribute.xml

@@ -43,6 +43,10 @@
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
+
+> [!NOTE]
+>  The .NET Framework, .NET Core, and .NET runtimes do not consistently enforce this attribute. Developers should not rely on this attribute to restrict access to an assembly's non-public members.
+
  Ordinarily, reflection methods, such a <xref:System.Type.GetMethod%2A?displayProperty=nameWithType> or <xref:System.Reflection.PropertyInfo.GetValue%2A?displayProperty=nameWithType>, can be used to get information about private members at runtime. When applies to an assembly, the <xref:System.Runtime.CompilerServices.DisablePrivateReflectionAttribute> attribute makes information about private members in that assembly's types unavailable to runtime reflection.  
   
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