Clarify concept of system messages and guarantee of delivery

[marcoscaceres]

On the mailing list, @mounirlamouri proposed the following:

Sure, I guess we should just change the end of that paragraph:

"System Messages are events sent by the system to an application which
has registered for it before. Those events are different from DOM events
in the sense that they are always originated by the system and that if
the targeted application isn't currently running, it will be started.
In addition, un-handled messages will stay in a queue."