Note that shutdown finalization is .NET Framework only (#2233)

Author: jkotas

xml/System/Environment.xml

@@ -1267,7 +1267,7 @@ Per-machine and per-user environment variables are not supported. A `target` val
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 ## Remarks  
- When the CLR unloads an application domain, it runs the finalizers on all objects that have a finalizer method in that application domain. When the CLR shuts down, it starts the finalizer thread on all objects that have a finalizer method. The <xref:System.Environment.HasShutdownStarted%2A> property returns `true` only after the finalizer thread has been started. When the property returns `true`, you can determine whether an application domain is being unloaded or the CLR itself is shutting down by calling the <xref:System.AppDomain.IsFinalizingForUnload%2A?displayProperty=nameWithType> method. This method returns `true` if finalizers are called because the application domain is unloading or `false` if the CLR is shutting down.  
+ **.NET Framework only**: When the CLR unloads an application domain, it runs the finalizers on all objects that have a finalizer method in that application domain. When the CLR shuts down, it starts the finalizer thread on all objects that have a finalizer method. The <xref:System.Environment.HasShutdownStarted%2A> property returns `true` only after the finalizer thread has been started. When the property returns `true`, you can determine whether an application domain is being unloaded or the CLR itself is shutting down by calling the <xref:System.AppDomain.IsFinalizingForUnload%2A?displayProperty=nameWithType> method. This method returns `true` if finalizers are called because the application domain is unloading or `false` if the CLR is shutting down.  
   
  The <xref:System.Environment.HasShutdownStarted%2A> property returns `false` if the finalizer thread has not been started.  
   
@@ -2343,4 +2343,4 @@ If `target` is <xref:System.EnvironmentVariableTarget.User?displayProperty=nameW
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