libstdc++: Fix Doxygen commands

Remove the bogus -D__allocator_base=std::__new_allocator macro
definition for Doxygen, because that's an alias template for C++11 and
later, not a macro.

Fix the @cond/@endcond pair that span the end of an @addtogroup group.
Add another @endcond inside the group, and another @cond after it.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Remove __allocator_base.
	* include/bits/allocator.h: Fix nesting of Doxygen commands.

libstdc++-v3/doc/doxygen/user.cfg.in

@@ -2407,7 +2407,6 @@ PREDEFINED             = __cplusplus=202002L \
                          _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED \
                          _GLIBCXX_HAVE_BUILTIN_LAUNDER \
 			 "_GLIBCXX_DOXYGEN_ONLY(X)=X " \
-			 __allocator_base=std::__new_allocator \
 			 __exception_ptr=__unspecified__ \
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this

libstdc++-v3/include/bits/allocator.h

@@ -265,6 +265,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef _Tp value_type;
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
+  /// @endcond
 
   /// @} group allocator
 
@@ -278,6 +279,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Undefine.
 #undef __allocator_base
 
+  /// @cond undocumented
+
   // To implement Option 3 of DR 431.
   template<typename _Alloc, bool = __is_empty(_Alloc)>
     struct __alloc_swap