patch 8.2.3578: manipulating highlighting is complicated

Problem:    Manipulating highlighting is complicated.
Solution:   Add the hlget() and hlset() functions. (Yegappan Lakshmanan,
            closes #9039)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 4ea51f9..89bd542 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -925,6 +925,8 @@
 	getmatches()		get all matches defined by |matchadd()| and
 				the |:match| commands
 	hlexists()		check if a highlight group exists
+	hlget()			get highlight group attributes
+	hlset()			set highlight group attributes
 	hlID()			get ID of a highlight group
 	synID()			get syntax ID at a specific position
 	synIDattr()		get a specific attribute of a syntax ID