Document string concatenation

Fixes #420


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4331 d4fa192b-c00b-0410-8231-f00ffab90ce4

man/vcl.7so

@@ -60,6 +60,9 @@ Unlike C and Perl, the backslash (\e) character has no special meaning
 in strings in VCL, which use the (%xx) escape mechanism just like URLs,
 so it can be freely used in regular expressions without doubling.
 .Pp
+Strings are concatenated by just putting them one after each other
+without any operator in between.
+.Pp
 Assignments are introduced with the
 .Cm set
 keyword.