Document the 'B' option of app_chanspy.

(closes issue #12582)
Reported by: IgorG
Patches:
      app_chanspy_B_option.diff uploaded by IgorG (license 20)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@115290 65c4cc65-6c06-0410-ace0-fbb531ad65f3