libstdc++: Add comment to <charconv> explaining C++14 status

This header is intentionally valid in C++14 mode, because no conforming
C++14 program will try to include <charconv> and so it's OK to add new
(non-reserved in C++14) names to namespace std. However, other headers
must not include <charconv> transitively prior to C++17, so that we
don't add those non-reserved names without the user requesting it.

This adds a comment to the header explaining that.

	* include/std/charconv: Add comment.

libstdc++-v3/ChangeLog

 2020-02-17  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/charconv: Add comment.
+
 	PR libstdc++/92546 (partial)
 	* include/bits/random.h (uniform_random_bit_generator): Move definition
 	to <bits/uniform_int_dist.h>.

libstdc++-v3/include/std/charconv

@@ -31,6 +31,10 @@
 
 #pragma GCC system_header
 
+// As an extension we support <charconv> in C++14, but this header should not
+// be included by any other library headers in C++14 mode. This ensures that
+// the names defined in this header are not added to namespace std unless a
+// user explicitly includes <charconv> in C++14 code.
 #if __cplusplus >= 201402L
 
 #include <type_traits>