improve 2BCT documentation a bit (thanks Jared)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@103315 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/zapata.conf.sample

@@ -301,6 +301,10 @@ threewaycalling=yes
 ;
 ; For digital ports using ISDN PRI protocols:
 ;   Support switch-side transfer (called 2BCT, RLT or other names)
+;   This setting must be enabled on both ports involved, and the
+;   'facilityenable' setting must also be enabled to allow sending
+;   the transfer to the ISDN switch, since it sent in a FACILITY
+;   message.
 ;
 transfer=yes
 ;