[docs] subprocess.Popen missing info

[David Abrahams]

The docs for subprocess.Popen are missing at least one piece
of information that is crucial to using it for portable code.  That
is, on POSIX systems, the PATH environment variable is always used to
look up directory-less executable names, but on Windows, PATH is only
considered when shell=True.  

Please see http://bugs.python.org/issue8557

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