[libc++][NFC] Add a short description for __split_buffer

[Mordante]

I like this thanks! LGTM!
Please wait a bit with landing in case others want to have a look too.

[huixie90]

LGMT

libcxx/include/__split_buffer

 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// __split_buffer allocates a contiguous chunk of memory and stores objects in the range [__begin_, __end_).
+// It has uninitialized memory in the ranges  [__first_, __begin_) and [__end_, __end_cap_.first()). That allows
+// it to grow both in the front and back without having to move the data.
+
 template <class _Tp, class _Allocator = allocator<_Tp> >
 struct __split_buffer
 {
 private:
     __split_buffer(const __split_buffer&);
     __split_buffer& operator=(const __split_buffer&);
 public:
     typedef _Tp                                             value_type;