Document that relations work in reduction relations

As exemplified by the following program:

```racket
#lang racket
(require redex)
(define-language L)
(define-relation L
  [(r a)])
(define ⇒
  (reduction-relation
   L
   (--> any it-holds
        (judgment-holds (r any)))))
(apply-reduction-relation ⇒ (term a)) ;; => '(it-holds)
(apply-reduction-relation ⇒ (term b)) ;; => '()
```

Author: leafac

redex-doc/redex/scribblings/ref/reduction-relations.scrbl

@@ -34,6 +34,7 @@
                            (bind @#,ttpattern @#,tttterm)
                            (bind/hidden @#,ttpattern @#,tttterm)
                            (judgment-holds (judgment-form-id pat/term ...))
+                           (judgment-holds (relation-id @#,tttterm ...))
                            (side-condition/hidden racket-expression)]
                [shortcuts (code:line)
                           (code:line with shortcut ...)]