Add exception to GetObjectForIUnknown (#2931)

* Add exception to GetObjectForIUnknown

This method will throw if given a "null" `IntPtr`.

* Correct capitalization

* fix link

Author: JeremyKuhne

xml/System.Runtime.InteropServices/Marshal.xml

@@ -4479,6 +4479,7 @@ There is a difference in the behavior of the `GetLastWin32Error` method on .NET
 
  ]]></format>
         </remarks>
+        <exception cref="T:SystemArgumentNullException"><paramref name="pUnk" /> is <see cref="F:System.IntPtr.Zero" />.</exception>
         <permission cref="T:System.Security.SecurityCriticalAttribute">requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.</permission>
         <altmember cref="M:System.Runtime.InteropServices.Marshal.AddRef(System.IntPtr)" />
         <altmember cref="M:System.Runtime.InteropServices.Marshal.Release(System.IntPtr)" />
@@ -12094,4 +12095,4 @@ public static void ThrowExceptionForHR(interrorCode,IntPtrerrorInfo)
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>