Improve Goto and GotoIf related documentation

Improve Goto and GotoIf related documentation

Correct documentation on labeliftrue and labeliffalse parameters of
GotoIf() and update several other locations that use the same syntax.

(closes issue ASTERISK-20007)
Patch-by: Leif Madsen
Reported-by: WIMPy
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Merged revisions 369869 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369871 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369872 65c4cc65-6c06-0410-ace0-fbb531ad65f3