Add a note about termination during activation. (#1451)

Activation handlers may not all get a chance to run, especially in the
case of browser termination.  A Service Worker should be designed to
function properly, even if the activation handlers do not all complete
successfully.

Update Nightly and V1

docs/index.bs

@@ -2776,6 +2776,8 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
 
           Note: Once an active worker is activating, neither a runtime script error nor a force termination of the active worker prevents the active worker from getting activated.
 
+          Note: Make sure to design activation handlers to do non-essential work (like cleanup). This is because activation handlers may not all run to completion, especially in the case of browser termination during activation. A Service Worker should be designed to function properly, even if the activation handlers do not all complete successfully.
+
       1. Let |matchedClients| be a [=list=] of [=/service worker clients=] whose <a>creation URL</a> <a lt="Match Service Worker Registration">matches</a> |registration|'s [=service worker registration/scope url=].
       1. [=list/For each=] |client| of |matchedClients|, [=queue a task=] on |client|'s [=responsible event loop=], using the [=DOM manipulation task source=], to run the following substeps:
           1. Let |readyPromise| be |client|'s [=environment settings object/global object=]'s {{ServiceWorkerContainer}} object's [=ServiceWorkerContainer/ready promise=].

docs/v1/index.bs

@@ -2588,6 +2588,8 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
 
           Note: Once an active worker is activating, neither a runtime script error nor a force termination of the active worker prevents the active worker from getting activated.
 
+          Note: Make sure to design activation handlers to do non-essential work (like cleanup). This is because activation handlers may not all run to completion, especially in the case of browser termination during activation. A Service Worker should be designed to function properly, even if the activation handlers do not all complete successfully.
+
       1. Let |matchedClients| be a [=list=] of [=/service worker clients=] whose <a>creation URL</a> <a lt="Match Service Worker Registration">matches</a> |registration|'s [=service worker registration/scope url=].
       1. [=list/For each=] |client| of |matchedClients|, [=queue a task=] on |client|'s [=responsible event loop=], using the [=DOM manipulation task source=], to run the following substeps:
           1. Let |readyPromise| be |client|'s [=environment settings object/global object=]'s {{ServiceWorkerContainer}} object's [=ServiceWorkerContainer/ready promise=].