Add missing documentation to misdn.txt

(closes issue #14052)
Reported by: festr
Patches:
      misdn.txt.patch uploaded by festr (license 443)



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@162659 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/misdn.txt

@@ -135,7 +135,7 @@ Dial and Options String
 The dial string of chan_misdn got more complex, because we added more features,
 so the generic dial string looks like:
 
-mISDN/<port>|g:<group>/<extension>[/<OPTIONSSTRING>]
+mISDN/<port>[:bchannel]|g:<group>/<extension>[/<OPTIONSSTRING>]
 
 The Optionsstring looks Like:
 :<optchar><optarg>:<optchar><optarg>...