chan_sip: Clarify in sample docs how directmediapermit/-acl should be used

It said "restrict [...] which peers should be able to pass [audio]
to each other".

However, these settings are not global (for which you would expect
signaling IPs to be checked). These settings are available per peer
only, and the IPs being checked, are the RTP IPs.

Change-Id: I2a6c6cd7c2f5f30d1df4844e3e0308a077021660

configs/samples/sip.conf.sample

@@ -1032,7 +1032,9 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
                                 ; the device is actually behind NAT.
 
 ;directmediadeny=0.0.0.0/0      ; Use directmediapermit and directmediadeny to restrict
-;directmediapermit=172.16.0.0/16; which peers should be able to pass directmedia to each other
+;directmediapermit=172.16.0.0/16; which RTP source IPs should be able to pass directmedia to
+                                ; each other. Note that directmedia ACLs are not a global
+                                ; setting, but must be defined per peer.
                                 ; (There is no default setting, this is just an example)
                                 ; Use this if some of your phones are on IP addresses that
                                 ; can not reach each other directly. This way you can force