doxygen: score: adjust doc in stack.h to doxygen guidelines

Update #3706.

cpukit/include/rtems/score/stack.h

@@ -1,6 +1,8 @@
 /**
  * @file
  *
+ * @ingroup RTEMSScoreStack
+ *
  * @brief Information About the Thread Stack Handler
  *
  * This include file contains all information about the thread
@@ -31,10 +33,13 @@ extern "C" {
  *
  * @ingroup RTEMSScore
  *
+ * @brief Stack Handler
+ *
  * This handler encapsulates functionality which is used in the management
  * of thread stacks.
+ *
+ * @{
  */
-/**@{*/
 
 /**
  *  The following constant defines the minimum stack size which every