Document BLINDTRANSFER behavior change.

(issue ASTERISK-19322)

(closes issue ASTERISK-19875)
Reported by: call
........

Merged revisions 368469 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@368470 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE.txt

@@ -31,6 +31,12 @@ From 10.3 to 10.4:
 
 From 10.2 to 10.3:
 
+* The BLINDTRANSFER channel variable is deleted from a channel when it is
+  bridged to prevent subtle bugs in the parking feature.  The channel
+  variable is used by Asterisk internally for the Park application to work
+  properly.  If you were using it for your own purposes, copy it to your
+  own channel variable before the channel is bridged.
+
 * If no transport is specified in sip.conf, transport will default to UDP.
   Also, if multiple transport= lines are used, only the last will be used.