asyncio doc: document BaseSubprocessTransport.close() method

Modify also the get_pipe_transport() doc to mention explicitly the supported
file descriptors.
asyncio doc: protocol_factory of subprocess_exec() and subprocess_shell() must
instanciate a subclass of SubprocessProtocol
asyncio doc: rewrite subprocess doc

* add a new example using transport and protocol
* rewrite the example using streams to make it much simpler (remove error
  handling, use a simpler Python code)
* copy (and adapt) more documentation from the subprocess module:

  - add a note about Process.wait() deadlock
  - add a note about shell injection
  - etc.

* sort Process methods and attributes in the same order than subprocess.Popen
  methods and attributes, so the documentation looks closer
* list differences between Process and subprocess.Popen APIs