make note of newer methods (dotnet#8095)

Author: gewarren

xml/System.Threading/Mutex.xml

@@ -588,10 +588,11 @@ There was some other error. The `HResult` property may provide more information.
 ## Remarks  
  The <xref:System.Threading.Mutex.GetAccessControl%2A> method uses the following combination of flags (combined using the bitwise OR operation) to search for permissions: <xref:System.Security.AccessControl.AccessControlSections.Access?displayProperty=nameWithType>, <xref:System.Security.AccessControl.AccessControlSections.Owner?displayProperty=nameWithType>, and <xref:System.Security.AccessControl.AccessControlSections.Group?displayProperty=nameWithType>.  
   
- The user must have <xref:System.Security.AccessControl.MutexRights.ReadPermissions?displayProperty=nameWithType> to call this method, and the mutex must have been opened with <xref:System.Security.AccessControl.MutexRights.ReadPermissions?displayProperty=nameWithType>.  
-  
-   
-  
+ The user must have <xref:System.Security.AccessControl.MutexRights.ReadPermissions?displayProperty=nameWithType> to call this method, and the mutex must have been opened with <xref:System.Security.AccessControl.MutexRights.ReadPermissions?displayProperty=nameWithType>.
+
+> [!NOTE]
+> In .NET Core and .NET 5+, the equivalent method is <xref:System.Threading.ThreadingAclExtensions.GetAccessControl%2A>.
+
 ## Examples  
  The following code example demonstrates the cross-process behavior of a named mutex with access control security. The example uses the <xref:System.Threading.Mutex.OpenExisting%28System.String%29> method overload to test for the existence of a named mutex.  
   
@@ -937,10 +938,11 @@ There was some other error. The `HResult` property may provide more information.
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 ## Remarks  
- The user must have <xref:System.Security.AccessControl.MutexRights.ChangePermissions?displayProperty=nameWithType> rights to call this method, and the mutex must have been opened with <xref:System.Security.AccessControl.MutexRights.ChangePermissions?displayProperty=nameWithType>.  
-  
-   
-  
+ The user must have <xref:System.Security.AccessControl.MutexRights.ChangePermissions?displayProperty=nameWithType> rights to call this method, and the mutex must have been opened with <xref:System.Security.AccessControl.MutexRights.ChangePermissions?displayProperty=nameWithType>.
+
+> [!NOTE]
+> In .NET Core and .NET 5+, the equivalent method is <xref:System.Threading.ThreadingAclExtensions.SetAccessControl%2A>.
+
 ## Examples  
  The following code example demonstrates the cross-process behavior of a named mutex with access control security. The example uses the <xref:System.Threading.Mutex.OpenExisting%28System.String%29> method overload to test for the existence of a named mutex.