Document that on Unix, the 'cmd' argument to the os.popen2/3/4 and

Document that on Unix, the 'cmd' argument to the os.popen2/3/4 and
popen2.popen2/3/4 functions can be a sequence. All texts are a variation on the
following:

On \UNIX, \var{cmd} may be a sequence, in which case arguments will be passed
directly to the program without shell intervention (as with
\function{os.spawnv()}). If \var{cmd} is a string it will be passed to the shell
(as with \function{os.system()}).