patch 8.2.2490: 'wrap' option is always reset when starting diff mode

Problem:    'wrap' option is always reset when starting diff mode.
Solution:   Add the "followwrap" item in 'diffopt'. (Rick Howe, closes #7797)
diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index 0b4edb9..566411a 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -59,7 +59,7 @@
 	'scrollbind'	on
 	'cursorbind'	on
 	'scrollopt'	includes "hor"
-	'wrap'		off
+	'wrap'		off, or leave as-is if 'diffopt' includes "followwrap"
 	'foldmethod'	"diff"
 	'foldcolumn'	value from 'diffopt', default is 2
 
@@ -144,7 +144,7 @@
 	'scrollbind'	off
 	'cursorbind'	off
 	'scrollopt'	without "hor"
-	'wrap'		on
+	'wrap'		on, or leave as-is if 'diffopt' includes "followwrap"
 	'foldmethod'	"manual"
 	'foldcolumn'	0