Variable.rst: udpate doc

Documentation/Variable.rst

@@ -258,7 +258,8 @@ Default
 * Any string can be given to define a default value.
 * If a default value is given, it makes no sense to define more than one store: with a default value given, only the
   first store is considered.
-* If the default value contains a ':', that one needs to be escaped by \\
+* If the default value contains a ':', that one needs to be escaped by ``\``
+* For dedicated variables this value has a special meaning. E.g. ``{{randomUniq:V}}`` uses this as ``expire`` argument.
 
 .. _`variable-type-message-violate`: