vim-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 vim/vim#7269,
            closes vim/vim#6714)
59d8e56e04

Include minimal test_buffer.vim from patch 8.2.0243 for Test_balt().
Add entry for :balt to runtime/doc/index.txt from vim/vim#7819.

src/nvim/ex_docmd.c

@@ -7271,9 +7271,7 @@ static void ex_find(exarg_T *eap)
   }
 }
 
-/*
- * ":edit", ":badd", ":visual".
- */
+/// ":edit", ":badd", ":balt", ":visual".
 static void ex_edit(exarg_T *eap)
 {
   do_exedit(eap, NULL);
@@ -7355,6 +7353,7 @@ do_exedit(
                 // After a split we can use an existing buffer.
                 + (old_curwin != NULL ? ECMD_OLDBUF : 0)
                 + (eap->cmdidx == CMD_badd ? ECMD_ADDBUF : 0)
+                + (eap->cmdidx == CMD_balt ? ECMD_ALTBUF : 0)
                 , old_curwin == NULL ? curwin : NULL) == FAIL) {
       // Editing the file failed.  If the window was split, close it.
       if (old_curwin != NULL) {
@@ -8725,7 +8724,7 @@ ssize_t find_cmdline_var(const char_u *src, size_t *usedlen)
  * Evaluate cmdline variables.
  *
  * change '%'	    to curbuf->b_ffname
- *	  '#'	    to curwin->w_altfile
+ *	  '#'	    to curwin->w_alt_fnum
  *	  '<cword>' to word under the cursor
  *	  '<cWORD>' to WORD under the cursor
  *	  '<cexpr>' to C-expression under the cursor