chan_sip: Note change in behavior to how directmediapermit/deny ACL works

r366547 introduced a change to the directmedia ACL for chan_sip which
modified the behavior significantly. Prior to the patch, this option would
bridge peers with directmedia if a peer's IP address matched its own
directmedia ACL. After that patch, the peer would check the bridged peer's
ACL instead. This change has been present since 1.8.14.0. That patched failed
to document the change in Upgrade.txt, so this patch adds mention of that
change to UPGRADE.txt (UPGRADE-1.8.txt in newer branches)

(issue AST-876)
........

Merged revisions 372471 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@372472 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE-1.8.txt

@@ -18,6 +18,11 @@
 ===
 ===========================================================
 
+From 1.8.13 to 1.8.14:
+* permitdirectmedia/denydirectmedia now controls whether peers can be
+  bridged via directmedia by comparing the ACL to the bridging peer's
+  address rather than its own address.
+
 From 1.8.12 to 1.8.13:
 * The complex processor detection and optimization has been removed from
   the makefile in favor of using native optimization suppport when available.