document the return value of gettag()

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@25063 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -10778,7 +10778,11 @@ static int attempt_transfer(struct sip_pvt *p1, struct sip_pvt *p2)
 	return 0;
 }
 
-/*! \brief Get tag from packet */
+/*! \brief Get tag from packet 
+ *
+ * \return Returns the pointer to the provided tag buffer,
+ *         or NULL if the tag was not found.
+ */
 static const char *gettag(const struct sip_request *req, char *header, char *tagbuf, int tagbufsize) 
 {
 	const char *thetag;