Clarify 'Values of Correct Type' rule relates to literals

spec/Section 5 -- Validation.md

@@ -1303,15 +1303,26 @@ 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}.
+  - {value} must be coercible to {type} (with the assumption that any
+    {variableUsage} nested within {value} will represent a runtime value
+    suitable for usage in its position).
 
 **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.
 
+A {ListValue} or {ObjectValue} may nest additional Input Values, some of which
+may be a {variableUsage}. The
+[All Variable Usages Are Allowed](#sec-All-Variable-Usages-Are-Allowed)
+validation rule asserts that each {variableUsage} is of a type allowed in that
+position, and the [Coercing Variable Values](#sec-Coercing-Variable-Values)
+algorithm will ensure runtime values of these variables coerce correctly, thus
+we can assume the runtime value of each {variableUsage} will be suitable for
+usage in its position.
+
 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
 provided for, and the type of a variable definition a default value is provided