Add documentation to the ability to retrieve the source port of a SIP call.

(belongs with r419970)

ASTERISK-24040 #close
Patches:
	func_channel.c.diff uploaded by dtryba

Review: https://reviewboard.asterisk.org/r/3781/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@420144 f38db490-d61c-443f-a65b-d21fe96a405b