=doc #3596 Add paragraph about Agent locality

* Based on discussion on akka-user, Sept 6-9 2013. It seems appropriate
to mention that Agents are not remoteable, so that naive users (like me)
know to design for that.
* This change has not been locally built with Sphinx, but seems to be
small and safe.
Merge pull request #1720 from jducoeur/master

=doc #3596 Add paragraph about Agent locality