Fix missing doc tags found while fixing ASTERISK-18689

Add missing <variable></variable> tags in app_dial documentation.
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Merged revisions 348992 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 348993 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348994 65c4cc65-6c06-0410-ace0-fbb531ad65f3