[racket] Scribble: "Warning: some cross references may be broken due to undefined tags"

Working on Scribble docs for first PLaneT package. Can't reference
Racket symbols using `@racket[]'. Probably doing something dumb, but
I've compared my .scrbl to a few on PLaneT and can't see what I'm
doing wrong/differently.

Distilled example:

1. Given manual.scrbl >>>

#lang scribble/manual

@(require planet/scribble
          scribble/eval
          racket/sandbox
          (for-label racket)
          )

@racket[(listof (cons/c symbol? string?))]

<<<

2. Run Scribble >>>

$ scribble manual.scrbl
 [Output to manual.html]
Warning: some cross references may be broken due to undefined tags:
 (dep ((lib "racket/main.rkt") string?))
 (dep ((lib "racket/private/pre-base.rkt") string?))
 (dep ((lib "racket/contract/base.rkt") cons/c))
 (dep ((lib "racket/private/base.rkt") symbol?))
 (dep ((lib "racket/base.rkt") symbol?))
 (dep ((lib "racket/main.rkt") cons/c))
 (dep ((quote #%kernel) string?))
 (dep ((lib "racket/contract.rkt") listof))
 (dep ((lib "racket/contract/base.rkt") listof))
 (dep ((quote #%kernel) symbol?))
 (dep ((lib "racket/base.rkt") string?))
 (dep ((lib "racket/contract/private/misc.rkt") listof))
 (dep ((lib "racket/contract.rkt") cons/c))
 (dep ((lib "racket/main.rkt") symbol?))
 (dep ((lib "racket/contract/private/misc.rkt") cons/c))
 (dep ((lib "racket/private/base.rkt") string?))
 (dep ((lib "racket/private/pre-base.rkt") symbol?))
 (dep ((lib "racket/main.rkt") listof))

<<<

3. Furthermore, viewing the resulting manual.html file in browser
shows them underlined in red as "bad links".

4. Scratch head and feel like a n%b.

5. Post here.

6. Thank folks in advance for any help.