[Varnish] #420: Document string concatenation

[Varnish]

#420: Document string concatenation
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 Reporter:  perbu          |        Owner:       
     Type:  defect         |       Status:  new  
 Priority:  normal         |    Milestone:       
Component:  documentation  |      Version:  trunk
 Severity:  normal         |   Resolution:       
 Keywords:                 |  
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Comment (by phk):

 Unfortunately, this is not (yet) universally the case in VCL, I need to go
 through and make it possible wherever a string is expected.

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Ticket URL: <http://varnish.projects.linpro.no/ticket/420#comment:1>
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