[docs] os.popen & os.system lack shell-related security warnings (issue 21557)

[pybugs at rebertia.com]

http://bugs.python.org/review/21557/diff/13339/Doc/library/os.rst
File Doc/library/os.rst (right):

http://bugs.python.org/review/21557/diff/13339/Doc/library/os.rst#newcode2913
Doc/library/os.rst:2913: For greater safety, consider using
:mod:`subprocess` with
On 2014/12/03 19:05:13, demian wrote:
> This should point to subprocess.Popen
> (https://docs.python.org/2/library/subprocess.html#subprocess.Popen)
rather than
> the module, no?

The subprocess module's convenience functions (subprocess.call(), etc.)
are also reasonable alternatives, so I just referred to the entire
subprocess module generally.

http://bugs.python.org/review/21557/