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

[demianbrecht at gmail.com]

I stand corrected, the patched docs do look good and are consistent with
the warning in "Frequently Used Arguments" section in the subprocess
module docs. Just a minor point about linking to subprocess.Popen rather
than the subprocess module.


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#newcode2905
Doc/library/os.rst:2905: .. warning::
Fair points. I also hadn't previously seen the warning in the frequently
used arguments section
(https://docs.python.org/2/library/subprocess.html#frequently-used-arguments)
and do like that this is consistent with it.

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
This should point to subprocess.Popen
(https://docs.python.org/2/library/subprocess.html#subprocess.Popen)
rather than the module, no?

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