docs: mention the behavior of scan using regex with capturing groups (#3240)

tests/manonig.test

@@ -47,6 +47,12 @@ scan("c")
 "c"
 "c"
 
+scan("(a+)(b+)")
+"abaabbaaabbb"
+["a","b"]
+["aa","bb"]
+["aaa","bbb"]
+
 split(", *"; null)
 "ab,cd, ef"
 ["ab","cd","ef"]