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: http://svn.asterisk.org/svn/asterisk/trunk@394304 f38db490-d61c-443f-a65b-d21fe96a405b
Remove redundant code in dns.c

Peter J Philipp pointed out that there are two checks that ensure that len is
not less than 0. If len is less than 0, the function returns. Having both of
them is clearly redundant.

This removes the second and attempts to clarify (slightly) the error condition.

(closes issue ASTERISK-21772)
Reported by: Peter J Philipp


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@394305 f38db490-d61c-443f-a65b-d21fe96a405b