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