Clarify ThreadAbortException on Core (dotnet#2823)

* Clarify ThreadAbortException on Core Call out that while the exception exists on .NET Core, it will never get thrown. * Update xml/System.Threading/ThreadAbortException.xml Co-Authored-By: Maira Wenzel <mairaw@microsoft.com>

Author: JeremyKuhne

xml/System.Threading/ThreadAbortException.xml

@@ -42,6 +42,8 @@
 ## Remarks 
  When a call is made to the <xref:System.Threading.Thread.Abort%2A> method to destroy a thread, the common language runtime throws a <xref:System.Threading.ThreadAbortException>. <xref:System.Threading.ThreadAbortException> is a special exception that can be caught, but it will automatically be raised again at the end of the `catch` block. When this exception is raised, the runtime executes all the `finally` blocks before ending the thread. Because the thread can do an unbounded computation in the `finally` blocks or call <xref:System.Threading.Thread.ResetAbort%2A?displayProperty=nameWithType> to cancel the abort, there is no guarantee that the thread will ever end. If you want to wait until the aborted thread has ended, you can call the <xref:System.Threading.Thread.Join%2A?displayProperty=nameWithType> method. <xref:System.Threading.Thread.Join%2A> is a blocking call that does not return until the thread actually stops executing. 
  
+**.NET Core only:** Even though this type exists in .NET Core, since <xref:System.Threading.Thread.Abort%2A> is not supported, the common language runtime won't ever throw <xref:System.Threading.ThreadAbortException>.
+
 > [!NOTE]
 > When the common language runtime (CLR) stops background threads after all foreground threads in a managed executable have ended, it does not use <xref:System.Threading.Thread.Abort%2A?displayProperty=nameWithType>. Therefore, you cannot use <xref:System.Threading.ThreadAbortException> to detect when background threads are being terminated by the CLR. 
  
@@ -137,4 +139,4 @@ Main ending.
  </Docs>
  </Member>
  </Members>
-</Type>
+</Type>