runtime(doc): clarify syntax vs matching mechanism

fixes: #14643

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -9980,6 +9980,10 @@ synconcealed({lnum}, {col})				*synconcealed()*
 			synconcealed(lnum, 5)   [1, 'X', 2]
 			synconcealed(lnum, 6)   [0, '', 0]
 
+		Note: Doesn't consider |matchadd()| highlighting items,
+		since syntax and matching highlighting are two different
+		mechanisms |syntax-vs-match|.
+
 
 synstack({lnum}, {col})					*synstack()*
 		Return a |List|, which is the stack of syntax items at the