Clarify that siblings of error propagation must not be added to the errors list

Author: benjie

spec/Section 6 -- Execution.md

@@ -606,7 +606,9 @@ entire list in the case of a list position), either resolving to {null} if
 allowed or being further propagated to a parent response position.
 
 If this occurs, any sibling response positions which have not yet executed or
-have not yet yielded a value may be cancelled to avoid unnecessary work.
+have not yet yielded a value may be cancelled to avoid unnecessary work. Any
+errors raised by these sibling response positions must not be added to the
+{"errors"} list in the _execution result_.
 
 Note: See [Handling Execution Errors](#sec-Handling-Execution-Errors) for more
 about this behavior.
@@ -922,8 +924,13 @@ result_.
 
 If a _response position_ resolves to {null} because of an execution error which
 has already been added to the {"errors"} list in the _execution result_, the
-{"errors"} list must not be further affected. That is, only one error should be
-added to the errors list per _response position_.
+{"errors"} list must not be further affected. If a sibling field of a response
+position that triggered error propagation also raises an error, this additional
+error must not be added to the {"errors"} list.
+
+Note: The result of the above is that exactly one error must be added to the
+{"errors"} list for each {null} appearing in the _execution response_ because of
+an error (including due to error propagation).
 
 Since `Non-Null` response positions cannot be {null}, execution errors are
 propagated to be handled by the parent _response position_. If the parent