Clarify 'Values of Correct Type' rule does not apply to variables.

Author: benjie

spec/Section 5 -- Validation.md

@@ -1303,14 +1303,16 @@ fragment resourceFragment on Resource {
 
 **Formal Specification**
 
-- For each input Value {value} in the document:
+- For each literal input Value {value} in the document:
   - Let {type} be the type expected in the position {value} is found.
   - {value} must be coercible to {type}.
 
 **Explanatory Text**
 
 Literal values must be compatible with the type expected in the position they
-are found as per the coercion rules defined in the Type System chapter.
+are found as per the coercion rules defined in the Type System chapter. Variable
+values are handled by the rule
+[All Variable Usages Are Allowed](#sec-All-Variable-Usages-Are-Allowed).
 
 The type expected in a position includes the type defined by the argument a
 value is provided for, the type defined by an input object field a value is