libstdc++: Fix description of std::ios::trunc (PR 92886)

	PR libstdc++/92886
	* include/bits/ios_base.h (std::ios_base::trunc): Fix comment.

From-SVN: r279175

libstdc++-v3/ChangeLog

 2019-12-10  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/92886
+	* include/bits/ios_base.h (std::ios_base::trunc): Fix comment.
+
 	* include/std/complex (__cpp_lib_constexpr_complex): Define.
 	* include/std/version (__cpp_lib_constexpr_complex): Likewise.
 	* testsuite/26_numerics/complex/1.cc: New test.

libstdc++-v3/include/bits/ios_base.h

@@ -445,7 +445,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     /// Open for output.  Default for @c ofstream and fstream.
     static const openmode out =		_S_out;
 
-    /// Open for input.  Default for @c ofstream.
+    /// Truncate an existing stream when opening.  Default for @c ofstream.
     static const openmode trunc =	_S_trunc;
 
     // 27.4.2.1.5  Type ios_base::seekdir