patch 8.2.3712: cannot use Vim9 lambda for 'tagfunc'

Problem:    Cannot use Vim9 lambda for 'tagfunc'.
Solution:   Make it work, add more tests. (Yegappan Lakshmanan, closes #9250)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 6c589d1..b0000ad 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -379,7 +379,15 @@
 	set opfunc=MyOpFunc
 	set opfunc=function('MyOpFunc')
 	set opfunc=funcref('MyOpFunc')
-	let &opfunc = "{t -> MyOpFunc(t)}"
+	set opfunc={a\ ->\ MyOpFunc(a)}
+	" set using a funcref variable
+	let Fn = function('MyTagFunc')
+	let &tagfunc = string(Fn)
+	" set using a lambda expression
+	let &tagfunc = "{t -> MyTagFunc(t)}"
+	" set using a variable with lambda expression
+	let L = {a, b, c -> MyTagFunc(a, b , c)}
+	let &tagfunc = string(L)
 <
 
 Setting the filetype