Clarifying that signal state is an instruction that may be ignored.

dom.bs

@@ -1548,6 +1548,11 @@ Worker).
 The <dfn attribute for=AbortSignal>aborted</dfn> attribute's getter must return true if the object's
 [=AbortSignal/aborted flag=] is set, and false otherwise.
 
+<div class=note>
+ A signal's state (such as <a for=AbortSignal>aborted</a> and the accompanying "abort" event)
+ represent the controller's wishes, but an API listening to a signal may choose to ignore it. For
+ instance, an "abort" signal may be ignored if the operation has already completed.
+</div>
 
 <h2 id=nodes>Nodes</h2>