Make onUpdateAvailable documentation clearer.

extensions/common/api/runtime.json

Index: extensions/common/api/runtime.json
diff --git a/extensions/common/api/runtime.json b/extensions/common/api/runtime.json
index 60095768452edc43124403a8a6919484a4717525..a5a32555750aa2628e45f99e84725605db44a4c8 100644
--- a/extensions/common/api/runtime.json
+++ b/extensions/common/api/runtime.json
@@ -377,7 +377,7 @@
       {
         "name": "onUpdateAvailable",
         "type": "function",
-        "description": "Fired when an update is available, but isn't installed immediately because the app is currently running. If you do nothing, the update will be installed the next time the background page gets unloaded, if you want it to be installed sooner you can explicitly call chrome.runtime.reload().",
+        "description": "Fired when an update is available, but isn't installed immediately because the app is currently running. If you do nothing, the update will be installed the next time the background page gets unloaded, if you want it to be installed sooner you can explicitly call chrome.runtime.reload(). If your extension is using a persistent background page, the background page of course never gets unloaded, so unless you call chrome.runtime.reload() manually in response to this event the update will not get installed until the next time chrome itself restarts. If no handlers are listening for this event, and your extension has a persistent background page, it behaves as if chrome.runtime.reload() is called in response to this event.",
         "parameters": [
           {
             "type": "object",