UPGRADE: Add a note about non-functionality

Add a note that the "retry on 403 response to REGISTER" for chan_sip is
non-functional in the versions in which it was first introduced.
........

Merged revisions 405088 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@405089 f38db490-d61c-443f-a65b-d21fe96a405b