Added more comments on pjsua_msg_data

git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@1362 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/pjsip/include/pjsua-lib/pjsua.h b/pjsip/include/pjsua-lib/pjsua.h
index 4b7c8c0..8c1d5b4 100644
--- a/pjsip/include/pjsua-lib/pjsua.h
+++ b/pjsip/include/pjsua-lib/pjsua.h
@@ -1070,7 +1070,10 @@
 typedef struct pjsua_msg_data
 {
     /**
-     * Additional message headers as linked list.
+     * Additional message headers as linked list. Application can add
+     * headers to the list by creating the header, either from the heap/pool
+     * or from temporary local variable, and add the header using
+     * linked list operation. See pjsip_apps.c for some sample codes.
      *
      * \par Python:
      * This field is implemented as string linked-list in Python, where each