Add execution logic referencing the directive

Reflects the implementation in https://github.com/graphql/graphql-js/pull/4192/files#diff-1d705c6a4c73cd3ce46190029e75abd3015b49de3ec250357dcf9b44a35cb7d0

Author: benjie

spec/Section 6 -- Execution.md

@@ -792,14 +792,19 @@ added to the {"errors"} list in the response, the {"errors"} list must not be
 further affected. That is, only one error should be added to the errors list per
 field.
 
-Since `Non-Null` type fields cannot be {null}, field errors are propagated to be
+If the operation provides the `@disableErrorPropagation` directive then
+`Non-Null` type fields will become {null} if an error occurs.
+
+If the operation does not provide the `@disableErrorPropagation` directive then
+`Non-Null` type fields cannot be {null}, and field errors are propagated to be
 handled by the parent field. If the parent field may be {null} then it resolves
 to {null}, otherwise if it is a `Non-Null` type, the field error is further
 propagated to its parent field.
 
 If a `List` type wraps a `Non-Null` type, and one of the elements of that list
 resolves to {null}, then the entire list must resolve to {null}. If the `List`
-type is also wrapped in a `Non-Null`, the field error continues to propagate
+type is also wrapped in a `Non-Null` and the operation does not provide the
+`@disableErrorPropagation` directive, the field error continues to propagate
 upwards.
 
 If all fields from the root of the request to the source of the field error