LWG-3880 Clarify `operator+=` complexity for `{chunk,stride}_view::iterator`

[StephanTLavavej]

LWG-3880 Clarify operator+= complexity for {chunk,stride}_view::iterator

This appears to be related to #2995 cited in these operators:

STL/stl/inc/ranges

Lines 6232 to 6257 in 9ae1b3f

	constexpr _Iterator& operator+=(const difference_type _Off) /* not strengthened, see _RANGES advance */
	requires random_access_range<_Base>
	{
	if (_Off > 0) {
	#if _ITERATOR_DEBUG_LEVEL != 0
	_STL_VERIFY(_Count <= (numeric_limits<difference_type>::max)() / _Off,
	"cannot advance chunk_view iterator past end (integer overflow)");
	if constexpr (sized_sentinel_for<_Base_sentinel, _Base_iterator>) {
	_STL_VERIFY(_End - _Current > _Count * (_Off - 1), //
	"cannot advance chunk_view iterator past end");
	}
	#endif // _ITERATOR_DEBUG_LEVEL != 0
	// Per to-be-filed LWG issue (See GH-2995)
	if constexpr (sized_sentinel_for<_Base_sentinel, _Base_iterator>) {
	_Missing = _RANGES advance(_Current, _Count * _Off, _End);
	} else {
	_Current += static_cast<difference_type>(_Count * (_Off - 1));
	_Missing = _RANGES advance(_Current, _Count, _End);
	}
	} else if (_Off < 0) {
	_Current += static_cast<difference_type>(_Count * _Off + _Missing);
	_Missing = 0;
	}
	return *this;
	}

STL/stl/inc/ranges

Lines 7182 to 7207 in 9ae1b3f

	constexpr _Iterator& operator+=(const difference_type _Off)
	requires random_access_range<_Base>
	{
	if (_Off > 0) {
	#if _ITERATOR_DEBUG_LEVEL != 0
	_STL_VERIFY(_Stride <= (numeric_limits<difference_type>::max)() / _Off,
	"cannot advance stride_view iterator past end (integer overflow)");
	if constexpr (sized_sentinel_for<_Base_sentinel, _Base_iterator>) {
	_STL_VERIFY(_End - _Current > _Stride * (_Off - 1), //
	"cannot advance stride_view iterator past end");
	}
	#endif // _ITERATOR_DEBUG_LEVEL != 0
	// Per to-be-filed LWG issue (See GH-2995)
	if constexpr (sized_sentinel_for<_Base_sentinel, _Base_iterator>) {
	_Missing = _RANGES advance(_Current, _Stride * _Off, _End);
	} else {
	_Current += static_cast<difference_type>(_Stride * (_Off - 1));
	_Missing = _RANGES advance(_Current, _Stride, _End);
	}
	} else if (_Off < 0) {
	_Current += static_cast<difference_type>(_Stride * _Off + _Missing);
	_Missing = 0;
	}
	return *this;
	}

I haven't determined whether we can simply eliminate these comments, or if further changes are required to conform to the resolution of LWG-3880.