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/diff.txt b/runtime/doc/diff.txt
index 6b638fa..60ebb4f 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -1,4 +1,4 @@
-*diff.txt*      For Vim version 8.1.  Last change: 2019 Feb 27
+*diff.txt*      For Vim version 8.1.  Last change: 2019 May 04
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -16,8 +16,6 @@
 4. Copying diffs		|copy-diffs|
 5. Diff options			|diff-options|
 
-{not in Vi}
-
 ==============================================================================
 1. Starting diff mode					*start-vimdiff*