[docs] asyncio.docs: Document subprocess_exec and subprocess_shell (issue 20694)

[yselivanov.ml at gmail.com]

http://bugs.python.org/review/20694/diff/11113/Doc/library/asyncio-eventloop.rst
File Doc/library/asyncio-eventloop.rst (right):

http://bugs.python.org/review/20694/diff/11113/Doc/library/asyncio-eventloop.rst#newcode505
Doc/library/asyncio-eventloop.rst:505: *shell*, which should not be
specified at all.
OK, please review the final patch before I commit it.

http://bugs.python.org/review/20694/