patch 8.2.1703: ":highlight clear" does not restore default link

Problem:    ":highlight clear" does not restore default link.
Solution:   Remember the default link and restore it. (Antony Scriven,
            closes #6970, closes #4405)
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 9c56567..1d4dd1f 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -4809,6 +4809,7 @@
 			highlighting for groups added by the user!
 			Uses the current value of 'background' to decide which
 			default colors to use.
+			If there was a default link, restore it. |:hi-link|
 
 :hi[ghlight] clear {group-name}
 :hi[ghlight] {group-name} NONE