"threadsafe"

Rewritten/updated sections:

* Event Loop APIs
* Transports & Protocols
* Streams
* Exceptions
* Policies
* Queues
* Subprocesses
* Platforms

* Replace 'yield from' to 'await' in asyncio docs

* Fix docstrings

The new -X dev command line option now also enables asyncio debug
mode.

"not only is it .." is the correct form, as opposed to:
"not only it is ..."

Patch by Julien Palard.

Patch by Benjamin Hodgson.

Patch by Martin Panter.

* dev: mention that the logging must be configured at DEBUG level
* streams: drain() has no more a strange return value, it's just
  a standard coroutine

- Document the new create_task() method
- "Hide" the Task class: point to the create_task() method for interoperability
- Rewrite the documentation of the Task class
- Document the "Pending task destroyed"
- Update output in debug mode of examples in the dev section
- Replace Task() with create_task() in examples

env var), document the default debug mode

Add also a PYTHONASYNCIODEBUG environment variable to debug coroutines since
Python startup, to be able to debug coroutines defined directly in the asyncio
module.

env var), document the default debug mode

Add also a PYTHONASYNCIODEBUG environment variable to debug coroutines since
Python startup, to be able to debug coroutines defined directly in the asyncio
module.

Improve also the "Logging" section