patch 8.1.1457: cannot reuse a buffer when loading a screen dump

Problem:    Cannot reuse a buffer when loading a screen dump.
Solution:   Add the "bufnr" option.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 844b68e..c095d58 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -9558,6 +9558,11 @@
 		   "curwin"	     use the current window, do not split the
 				     window; fails if the current buffer
 				     cannot be |abandon|ed
+		   "bufnr"	     do not create a new buffer, use the
+				     existing buffer "bufnr".  This buffer
+				     must have been previously created with
+				     term_dumpdiff() or term_dumpload() and
+				     visible in a window.
 		   "norestore"	     do not add the terminal window to a
 				     session file