The output of "python3.5 --help" says:

    -u : unbuffered binary stdout and stderr, stdin always buffered;
         also PYTHONUNBUFFERED=x
         see man page for details on internal buffering relating to '-u'

If you look at the man page as instructed then you'll see a clearer
explanation:

    -u   Force  the  binary  I/O  layers  of  stdout  and  stderr  to  be
         unbuffered.  stdin is always buffered.  The text I/O layer  will
         still be line-buffered.

For example, if you try this:

    python3.5 -uc 'import sys,time;w=sys.stdout.buffer.write;w(b"a");time.sleep(1);w(b"b");'

then you'll see that the binary output is indeed unbuffered as
documented.

The output of --help is trying to abbreviate this explanation, but I
think it's abbreviated too much. The explanation from the man page
seems clear to me, and is only a little longer, so I suggest changing
the --help output to match the man page.