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/options.txt b/runtime/doc/options.txt
index 74bab42..f25b1ba 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2677,6 +2677,8 @@
 		foldcolumn:{n}	Set the 'foldcolumn' option to {n} when
 				starting diff mode.  Without this 2 is used.
 
+		followwrap	Follow the 'wrap' option and leave as it is.
+
 		internal	Use the internal diff library.  This is
 				ignored when 'diffexpr' is set.  *E960*
 				When running out of memory when writing a