Document that default values in records must not use variables

Closes #8166

system/doc/reference_manual/ref_man_records.md

@@ -36,11 +36,14 @@ given an optional default value. If no default value is supplied, `undefined` is
 used.
 
 ```erlang
--record(Name, {Field1 [= Value1],
+-record(Name, {Field1 [= Expr1],
                ...
-               FieldN [= ValueN]}).
+               FieldN [= ExprN]}).
 ```
 
+The default value for a field is an arbitrary expression, except that it must
+not use any variables.
+
 A record definition can be placed anywhere among the attributes and function
 declarations of a module, but the definition must come before any usage of the
 record.