av_tempfile: document umask() issue

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/file.h

@@ -57,6 +57,9 @@ void av_file_unmap(uint8_t *bufptr, size_t size);
  * *prefix can be a character constant; *filename will be allocated internally.
  * @return file descriptor of opened file (or -1 on error)
  * and opened file name in **filename.
+ * @note On very old libcs it is necceary to set a secure umask before
+ *       calling this, av_tempfile() cant call umask itself as it is used in
+ *       libraries and could interfere with the calling application.
  */
 int av_tempfile(const char *prefix, char **filename, int log_offset, void *log_ctx);