Various small fixups to the multiprocessing docs, mostly fixing and

Various small fixups to the multiprocessing docs, mostly fixing and
enabling doctests that Sphinx can run, and fixing and disabling tests that
Sphinx can't run.  I hand checked every test not now marked as a doctest,
and all except the two that have open bug reports against them now work,
at least on Linux/trunk. (I did not look at the last example at all since
there was already an open bug).  I did not read the whole document with
an editor's eye, but I did fix a few things I noticed while working on
the tests.