Minor improvements to the threading introduction:

* Expand the example to show a join.

* Mention the use case of I/O running concurrent with a computational
  thread.

* Be a tad more forceful about recommending Queue over other approaches
  to synchonization.

* Eliminate discussion around having a single interpreter.  This is a
  more advanced discussion that belongs in the library reference and
  in a section on extending and embedding.