Update documentation for SIP_HEADER.

The SIP_HEADER function only works on the the initial SIP INVITE. The documentation was updated
in trunk, but not in 1.8 or 10, so I'm making them match.

(Closes issue ASTERISK-18640)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@338492 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -339,7 +339,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 	</application>
 	<function name="SIP_HEADER" language="en_US">
 		<synopsis>
-			Gets the specified SIP header.
+			Gets the specified SIP header from an incoming INVITE message.
 		</synopsis>
 		<syntax>
 			<parameter name="name" required="true" />