dialog: updated comments for api functions to get dlg var value

(cherry picked from commit 8548aaa8a798b939e041821848b2f05b963451d8)

src/modules/dialog/dlg_cb.h

@@ -54,13 +54,13 @@ typedef int (*register_dlgcb_f)(struct dlg_cell* dlg, int cb_types,
 typedef int (*set_dlg_variable_f)( struct dlg_cell* dlg,
                                    str* key,
                                    str* val);
-/* method to get a variable from a dialog */
+/* method to get a variable value reference from a dialog */
 typedef str* (*get_dlg_varref_f)( struct dlg_cell* dlg,
                                     str* key);
-/* method to get a variable from a dialog */
+/* method to get a variable value static-buffer duplicate from a dialog */
 typedef int (*get_dlg_varval_f)( struct dlg_cell* dlg,
                                     str* key, str* val);
-/* method to get a variable value duplicate from a dialog */
+/* method to get a variable value pkg-allocated duplicate from a dialog */
 typedef int (*get_dlg_vardup_f)( struct dlg_cell* dlg,
                                     str* key, str* val);