Underspecified behavior for Stream Finished Callback with Blocking Streams

[rossb]

The following problems exist with the specification of Stream Finished Callback (SFC) with Blocking Streams:

- There is no documentation saying whether it is permitted

- The SFC passes a userData ptr, but the specification for OpenStream?() says that userData is ignored for blocking streams (need to update documentation)

* Need to clean up the documentation and write a new test to test this case.

- also the documentation should be cleaned up to indicte that only one stream callback function can be registered with a stream at a time.

See the following mailing list post for discussion http://techweb.rfa.org/pipermail/portaudio/2007-September/007519.html