patch 8.2.2569: 'fillchars' "stl" and "stlnc" items must be single byte

Problem:    'fillchars' "stl" and "stlnc" items must be single byte.
Solution:   Accept multi-byte characters. (Christian Wellenbrock, Yegappan
            Lakshmanan, closes #7927)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 7074608..7fcdfbe 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3262,7 +3262,8 @@
 <	This is similar to the default, except that these characters will also
 	be used when there is highlighting.
 
-	for "stl" and "stlnc" only single-byte values are supported.
+	For "stl" and "stlnc" single-byte and multibyte characters are
+	supported.  But double-width characters are not supported.
 
 	The highlighting used for these items:
 	  item		highlight group ~