Update sip_request_call SIP dial string documentation.

This was missed when merging review r1859.
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Merged revisions 375074 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375078 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375079 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375080 65c4cc65-6c06-0410-ace0-fbb531ad65f3