Improve documentation for the SHELL() dialplan function.

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Merged revisions 370383 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 370384 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370385 65c4cc65-6c06-0410-ace0-fbb531ad65f3