patch 8.1.0253: saving and restoring window title does not always work

Problem:    Saving and restoring window title does not always work.
Solution:   Use the stack push and pop commands. (Kouichi Iwamoto,
            closes #3059)
diff --git a/runtime/doc/term.txt b/runtime/doc/term.txt
index 77cd933..4ade4a1 100644
--- a/runtime/doc/term.txt
+++ b/runtime/doc/term.txt
@@ -352,6 +352,10 @@
 	t_SH	set cursor shape				*t_SH* *'t_SH'*
 	t_RC	request terminal cursor blinking		*t_RC* *'t_RC'*
 	t_RS	request terminal cursor style			*t_RS* *'t_RS'*
+	t_ST	save window title to stack			*t_ST* *'t_ST'*
+	t_RT	restore window title from stack			*t_RT* *'t_RT'*
+	t_Si	save icon text to stack				*t_Si* *'t_Si'*
+	t_Ri	restore icon text from stack			*t_Ri* *'t_Ri'*
 
 Some codes have a start, middle and end part.  The start and end are defined
 by the termcap option, the middle part is text.