Fix cut-n-paste in leaf_paths; doc and test 'em

tests/all.test

@@ -440,6 +440,14 @@ path(.)
 42
 []
 
+[paths]
+[1,[[],{"a":2}]]
+[[0],[1],[1,0],[1,1],[1,1,"a"]]
+
+[leaf_paths]
+[1,[[],{"a":2}]]
+[[0],[1,1,"a"]]
+
 ["foo",1] as $p | getpath($p), setpath($p; 20), delpaths([$p])
 {"bar": 42, "foo": ["a", "b", "c", "d"]}
 "b"