avutil/file: fix av_tempfile() documentation

Found-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/file.h

@@ -55,7 +55,8 @@ void av_file_unmap(uint8_t *bufptr, size_t size);
  * Wrapper to work around the lack of mkstemp() on mingw.
  * Also, tries to create file in /tmp first, if possible.
  * *prefix can be a character constant; *filename will be allocated internally.
- * @return file descriptor of opened file (or -1 on error)
+ * @return file descriptor of opened file (or negative value corresponding to an
+ * AVERROR code on error)
  * and opened file name in **filename.
  * @note On very old libcs it is necessary to set a secure umask before
  *       calling this, av_tempfile() can't call umask itself as it is used in