patch 8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes #7269,
            closes #6714)
diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index 5f842c8..2a1c10b 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -1,4 +1,4 @@
-*windows.txt*   For Vim version 8.2.  Last change: 2020 Sep 02
+*windows.txt*   For Vim version 8.2.  Last change: 2020 Nov 07
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -1119,6 +1119,11 @@
 		line when the buffer is first entered.  Note that other
 		commands after the + will be ignored.
 
+						 *:balt*
+:balt [+lnum] {fname}
+		Like `:badd` and also set the alternate file for the current
+		window to {fname}.
+
 :[N]bd[elete][!]			*:bd* *:bdel* *:bdelete* *E516*
 :bd[elete][!] [N]
 		Unload buffer [N] (default: current buffer) and delete it from