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.