patch 8.2.2912: MS-Windows: most users expect using Unicode

Problem:    MS-Windows: most users expect using Unicode.
Solution:   Default 'encoding' to utf-8 on MS-Windows. (Ken Takata,
            closes #3907)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 4fbc014..2f69049 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2828,7 +2828,8 @@
 	|setcellwidths()| function to change the behavior.
 
 					*'encoding'* *'enc'* *E543*
-'encoding' 'enc'	string (default: "latin1" or value from $LANG)
+'encoding' 'enc'	string (default for MS-Windows: "utf-8",
+				otherwise: value from $LANG or "latin1")
 			global
 	Sets the character encoding used inside Vim.  It applies to text in
 	the buffers, registers, Strings in expressions, text stored in the