runtime(doc): add reference to matchbufline() at :h search()

related: #14173

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 941c6a7..7f71115 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -7980,6 +7980,7 @@
 		When a match has been found its line number is returned.
 		If there is no match a 0 is returned and the cursor doesn't
 		move.  No error message is given.
+		To get the matched string, use |matchbufline()|.
 
 		{flags} is a String, which can contain these character flags:
 		'b'	search Backward instead of forward