libstdc++: Improve comment about testsuite utilities

This fixes a typo and also explains why test_container is not a range
when used with output_iterator_wrapper or input_iterator_wrapper.

	* testsuite/util/testsuite_iterators.h: Improve comment.

From-SVN: r280146

libstdc++-v3/ChangeLog

 2020-01-10  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/util/testsuite_iterators.h: Improve comment.
+
 	* testsuite/25_algorithms/equal/deque_iterators/1.cc: Don't use C++11
 	initialization syntax.
 

libstdc++-v3/testsuite/util/testsuite_iterators.h

@@ -759,10 +759,13 @@ namespace __gnu_test
     using test_output_sized_range
       = test_sized_range<T, output_iterator_wrapper>;
 
-// test_container, test_range and test_sized_range do not own their elements,
-// so they all model std::ranges::safe_range. This file does not define
-// specializations of std::ranges::enable_safe_range, so that individual
-// test can decide whether or not to do so.
+// test_range and test_sized_range do not own their elements, so they model
+// std::ranges::safe_range.  This file does not define specializations of
+// std::ranges::enable_safe_range, so that individual tests can decide
+// whether or not to do so.
+// This is also true for test_container, although only when it has forward
+// iterators (because output_iterator_wrapper and input_iterator_wrapper are
+// not default constructible so do not model std::input_or_output_iterator).
 #endif // C++20
 } // namespace __gnu_test
 #endif // _TESTSUITE_ITERATORS