patch 8.1.1280: remarks about functionality not in Vi clutters the help

Problem:    Remarks about functionality not in Vi clutters the help.
Solution:   Move all info about what is new in Vim or already existed in Vi to
            vi_diff.txt.  Remove {not in Vi} remarks. (closes #4268) Add
            "noet" to the help files modeline.  Also include many other help
            file improvements.
diff --git a/runtime/doc/usr_21.txt b/runtime/doc/usr_21.txt
index 513dad0..86aadc2 100644
--- a/runtime/doc/usr_21.txt
+++ b/runtime/doc/usr_21.txt
@@ -1,4 +1,4 @@
-*usr_21.txt*	For Vim version 8.1.  Last change: 2012 Nov 02
+*usr_21.txt*	For Vim version 8.1.  Last change: 2019 Apr 25
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -263,7 +263,8 @@
 The windows that were open are restored, with the same position and size as
 before.  Mappings and option values are like before.
    What exactly is restored depends on the 'sessionoptions' option.  The
-default value is "blank,buffers,curdir,folds,help,options,winsize".
+default value is:
+"blank,buffers,curdir,folds,help,options,tabpages,winsize,terminal".
 
 	blank		keep empty windows
 	buffers		all buffers, not only the ones in a window
@@ -271,7 +272,9 @@
 	folds		folds, also manually created ones
 	help		the help window
 	options		all options and mappings
+	tabpages	all tab pages
 	winsize		window sizes
+	terminal	include terminal windows
 
 Change this to your liking.  To also restore the size of the Vim window, for
 example, use: >