Update "tube" document to reflect API

[jMyles]

@glyph points out in #82 that the docs are out-of-date insofar as linesToNumbersOrOperators is shown as being decorated @tube, but not @receiver.

On master, in docs/listings/rpn.py, linesToNumbersOrOperators is indeed shown as decorated @receiver(inputType=IFrame). In addition, tube.rst has :prepend: @tube in the entry for this include.

I have built the docs myself locally and the entry appears this way:

@tube
@receiver(inputType=IFrame)
def linesToNumbersOrOperators(line):
    from operator import add, mul
    try:
        yield int(line)
    except ValueError:
        if line == b'+':
            yield add
        elif line == b'*':
            yield mul

So, two questions:

1. How is it supposed to read? With only the @receiver decorator? Or the @tube decorator as well?

2. Any idea why the RTFD is out of date with master? Can I help get that on track?