Add note for temporary install of web extensions (#899)

Author: whimboo

index.bs

@@ -12223,6 +12223,8 @@ The [=remote end steps=] with |command parameters| are:
 
 </div>
 
+Note: Browsers may install the web extension only temporarily by default so
+that they will be automatically uninstalled during the next shutdown.
 
 #### The webExtension.uninstall Command #### {#command-webExtension-uninstall}