Document Exception.StackTrace nullability (#8739)

Author: colejohnson66

xml/System/Exception.xml

@@ -1435,7 +1435,7 @@ Stack Trace:
       </ReturnValue>
       <Docs>
         <summary>Gets a string representation of the immediate frames on the call stack.</summary>
-        <value>A string that describes the immediate frames of the call stack.</value>
+        <value>A string that describes the immediate frames of the call stack. If no stack trace is available (such as prior to stack unwinding from a <see langword="throw" /> statement), the value is <see langword="null" />.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[