Documentation: child_process.sparn options.stdio stream

doc/api/child_process.markdown

@@ -357,6 +357,8 @@ index corresponds to a fd in the child.  The value is one of the following:
    file, socket, or a pipe with the child process. The stream's underlying
    file descriptor is duplicated in the child process to the fd that 
    corresponds to the index in the `stdio` array.
+   Note that stream must have a valid file descriptor,
+   that means file must be opened.
 5. Positive integer - The integer value is interpreted as a file descriptor 
    that is is currently open in the parent process. It is shared with the child
    process, similar to how `Stream` objects can be shared.