Sibling errors should not be added after propagation

Author: benjie

spec/Section 6 -- Execution.md

@@ -605,8 +605,10 @@ execution position (the entire selection set in the case of a field, or the
 entire list in the case of a list position), either resolving to {null} if
 allowed or being further propagated to a parent execution position.
 
-If this occurs, any sibling _execution position_ which has not yet executed or
-has not yet yielded a value may be cancelled to avoid unnecessary work.
+If this occurs, any sibling _execution position_ or descendent thereof which has
+not yet executed or has not yet yielded a value may be cancelled to avoid
+unnecessary work. Errors from such positions should not be added to the
+{"errors"} list.
 
 Note: See [Handling Execution Errors](#sec-Handling-Execution-Errors) for more
 about this behavior.