Some of the docstrings already mention UnsupportedOperation. This patch updates the rest of the documentation. Also adds some tests to verify this on all the concrete classes I could think of. Some discoveries in the process:

* BufferedWriter.readable() and BufferedReader.writable() could return True depending on the underlying raw stream. Fixed to always return False.
* Removed a branch in a test case that assumed BufferedReader.close() did not call flush(), but never activated due to the above writable() bug
* seek(), tell() and truncate() do not raise UnsupportedOperation, despite seekable() == False, at least for a pipe. Adjusted doc strings.