small update to the g() option of app_voicemail to

note that gain changes only work on zap channels right now.
issue #12578 shows it's not clear right now.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@118417 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_voicemail.c

@@ -522,6 +522,7 @@ static char *descrip_vm =
 	"             the greeting.  Context defaults to the current context.\n"
 	"    g(#) - Use the specified amount of gain when recording the voicemail\n"
 	"           message. The units are whole-number decibels (dB).\n"
+	"           Only works on supported technologies, which is Zap only.\n"
 	"    s    - Skip the playback of instructions for leaving a message to the\n"
 	"           calling party.\n"
 	"    u    - Play the 'unavailable' greeting.\n"