Document ExternalIVR event tag collision

ExternalIVR uses the D tag for two different event types. This documents that
behavior and how to differentiate between the two cases. Also includes a minor
spelling fix and clarification

(closes issue #16211)
Reported by: thedavidfactor
Patches:
      externalivr.txt.20091109.1507.patch uploaded by thedavidfactor (license 903)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@229191 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/externalivr.txt

@@ -44,7 +44,7 @@ DTMF (and other) events
 
 All events will be newline-terminated strings.
 
-Events send to the child's stdin will be in the following format:
+Events sent to the child's stdin will be in the following format:
 
 tag,timestamp[,data]
 
@@ -57,9 +57,11 @@ A-D: DTMF event for keys A through D
 H: the channel was hung up by the connected party
 Z: the previous command was unable to be executed (file does not
 exist, etc.)
-T: the play list was interrupted (see below)
+T: the play list was interrupted (see S command below)
 D: a file was dropped from the play list due to interruption (the
-data element will be the dropped file name)
+data element will be the dropped file name) NOTE: this tag conflicts
+with the D DTMF event tag. The existence of the data element is used
+to differentiate between the two cases
 F: a file has finished playing (the data element will be the file
 name)