[Doc] Document hana::string instead of hana::String

example/string/string_c.cpp

@@ -0,0 +1,16 @@
+/*
+@copyright Louis Dionne 2015
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/assert.hpp>
+#include <boost/hana/core/is_a.hpp>
+#include <boost/hana/string.hpp>
+namespace hana = boost::hana;
+
+
+constexpr auto str = hana::string_c<'a', 'b', 'c', 'd', 'e', 'f'>;
+BOOST_HANA_CONSTANT_CHECK(hana::is_a<hana::String>(str));
+
+int main() { }