Clarify nested variable usages in Values of Correct Type

Author: benjie

spec/Section 5 -- Validation.md

@@ -1303,16 +1303,23 @@ fragment resourceFragment on Resource {
 
 **Formal Specification**
 
-- For each literal 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}.
+  - {value} must be coercible to {type} (with the assumption that any
+    {variableUsage} nested within {value} will represent a runtime value of the
+    referenced variable's 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. Variable
 values are handled by the rule
 [All Variable Usages Are Allowed](#sec-All-Variable-Usages-Are-Allowed).
+{ListValue} and {ObjectValue} may nest additional Input Values, some of which
+may be a {variableUsage}. Each nested {variableUsage} will be coerced during
+execution - see [Coercing Variable Values](#sec-Coercing-Variable-Values) - thus
+we assume their runtime value will coerce to the type of the referenced
+variable.
 
 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