Add explanation and warning about the "s" extension. (Hi Mike :-)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@36080 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/extensions.conf.sample

@@ -206,6 +206,12 @@ include => dundi-e164-switch
 ;
 ; ARG1 is the extension to Dial
 ;
+; Extension "s" is not a wildcard extension that matches "anything".
+; In macros, it is the start extension. In most other cases, 
+; you have to goto "s" to execute that extension.
+;
+; For wildcard matches, see above - all pattern matches start with
+; an underscore.
 exten => s,1,Goto(${ARG1},1)
 include => dundi-e164-lookup