clarify option description (issue #5351)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6705 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/voicemail.conf.sample

@@ -15,7 +15,8 @@ attach=yes
 ;maxmsg=100
 ; Maximum length of a voicemail message in seconds
 ;maxmessage=180
-; Minimum length of a voicemail message in seconds
+; Minimum length of a voicemail message in seconds for the message to be kept
+; The default is no minimum.
 ;minmessage=3
 ; Maximum length of greetings in seconds
 ;maxgreet=60