configs/samples/features.conf.sample: Document attended transfer DTMF options

The sample config was missing the configuration options for DTMF attended
transfer completion scenarios. The configuration options 'atxferabort',
'atxfercomplete', 'atxferthreeway', and 'atxferswap' are now documented in the
appropriate configuration file.

ASTERISK-24678 #close
Reported by: Niklas Larsson
patches:
  features.conf.sample.diff uploaded by Niklas Larsson (License 5068)
........

Merged revisions 430526 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430527 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/samples/features.conf.sample

@@ -28,7 +28,10 @@
                                 ; being kicked back to the original call.
 ;transferretrysound = "beep"    ; Sound to play when a transferer fails to dial a valid extension.
 ;transferinvalidsound = "beeperr" ; Sound to play when a transferer fails to dial a valid extension and is out of retries.
-
+;atxferabort = *1               ; cancel the attended transfer
+;atxfercomplete = *2            ; complete the attended transfer, dropping out of the call
+;atxferthreeway = *3            ; complete the attended transfer, but stay in the call. This will turn the call into a multi-party bridge
+;atxferswap = *4                ; swap to the other party. Once an attended transfer has begun, this options may be used multiple times
 
 ; Note that the DTMF features listed below only work when two channels have answered and are bridged together.
 ; They can not be used while the remote party is ringing or in progress. If you require this feature you can use