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/syntax.txt b/runtime/doc/syntax.txt
index 1c20659..9d4ea39 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -4849,6 +4849,7 @@
 :hi[ghlight] {group-name}
 			List one highlight group.
 
+						*highlight-clear*
 :hi[ghlight] clear	Reset all highlighting to the defaults.  Removes all
 			highlighting for groups added by the user!
 			Uses the current value of 'background' to decide which