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