Update AutoResetEvent.xml - clarified definition (#9566)

Clarified definition of AutoResetEvent to make initial definition more clear.

Author: Spharad

xml/System.Threading/AutoResetEvent.xml

@@ -55,7 +55,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Represents a thread synchronization event that, when signaled, resets automatically after releasing a single waiting thread. This class cannot be inherited.</summary>
+    <summary>Represents a thread synchronization event that, when signaled, releases one single waiting thread, and the event resets automatically. If no thread is waiting, the next thread that is set to the waiting state is immediately released, and the event resets automatically. This class cannot be inherited.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[