Clarify documentation for function PASSTHRU

It is not apparent to the average user that the PASSTHRU function should not
be passed as ${PASSTHRU(string)} but just as PASSTHRU(string) to functions
which take a variable name and not its contents.

This patch clarifies the behavior in the documentation and provides an example.

(closes issue ASTERISK-21717)
Reported by: Richard Miller
patches:
  func_strings.diff uploaded by Richard Miller (license 5685)
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Merged revisions 394302 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 394303 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394304 65c4cc65-6c06-0410-ace0-fbb531ad65f3