Update remarks for UnhandledException event (#11849)

Added clarification that handlers may be invoked multiple times if exceptions are thrown from different threads.

Author: AaronRobinsonMSFT

xml/System/AppDomain.xml

@@ -8821,7 +8821,10 @@ The friendly name of the default application domain is the file name of the proc
       </ReturnValue>
       <Docs>
         <summary>Occurs when an exception is not caught.</summary>
-        <remarks>For more information about this API, see <see href="/dotnet/fundamentals/runtime-libraries/system-appdomain-unhandledexception">Supplemental API remarks for UnhandledException</see>.</remarks>
+        <remarks>
+          Handlers may be invoked multiple times if exceptions are thrown from different threads.
+
+          For more information about this API, see <see href="/dotnet/fundamentals/runtime-libraries/system-appdomain-unhandledexception">Supplemental API remarks for UnhandledException</see>.</remarks>
         <example>
           <format type="text/markdown"><![CDATA[
 The following example demonstrates the <xref:System.AppDomain.UnhandledException> event. It defines an event handler, `MyHandler`, that is invoked whenever an unhandled exception is thrown in the default application domain. It then throws two exceptions. The first is handled by a **try/catch** block. The second is unhandled and invokes the `MyHandle` routine before the application terminates.