Default value coercion rules

Author: benjie

spec/Section 3 -- Type System.md

@@ -917,6 +917,10 @@ May yield the result:
 The type of an object field argument must be an input type (any type except an
 Object, Interface, or Union type).
 
+**Default Value Coercion**
+
+Where an argument specifies a default value, that value should be coerced
+according to the input coercion rules of the argument's type.
 
 ### Field Deprecation
 
@@ -1448,6 +1452,11 @@ define arguments or contain references to interfaces and unions, neither of
 which is appropriate for use as an input argument. For this reason, input
 objects have a separate type in the system.
 
+**Default Value Coercion**
+
+Where a field in an input object specifies a default value, that value should
+be coerced according to the input coercion rules of the field's type.
+
 **Result Coercion**
 
 An input object is never a valid result. Input Object types cannot be the return