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)
........

Merged revisions 394302 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@394303 f38db490-d61c-443f-a65b-d21fe96a405b