patch 8.1.1405: "highlight" option of popup windows not supported

Problem:    "highlight" option of popup windows not supported.
Solution:   Implement the "highlight" option.
diff --git a/runtime/doc/popup.txt b/runtime/doc/popup.txt
index af51885..cc9b7d1 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -322,8 +322,7 @@
 	wrap		TRUE to make the lines wrap (default TRUE)
 			{not implemented yet}
 	highlight	highlight group name to use for the text, stored in
-			'wincolor'
-			{not implemented yet}
+			the 'wincolor' option
 	border		list with numbers, defining the border thickness
 			above/right/below/left of the popup; an empty list
 			uses a border of 1 all around