On this page:
defsignature/ splice
4.2.5 Documenting Signatures

(defsignature id (super-id ...) pre-flow ...)
Defines a signature id that extends the super-id signatures. Any elements defined in decoded pre-flows – including forms, procedures, structure types, classes, interfaces, and mixins – are defined as members of the signature instead of direct bindings. These definitions can be referenced through sigelem instead of racket.

The decoded pre-flows inset under the signature declaration in the typeset output, so no new sections, etc. can be started.

(defsignature/splice id (super-id ...) pre-flow ...)
Like defsignature, but the decoded pre-flows are not typeset under the signature declaration, and new sections, etc. can be started in the pre-flows.

(signature-desc pre-flow ...)  any/c
  pre-flow : pre-flow?
Produces an opaque value that defsignature recognizes to outdent in the typeset form. This is useful for text describing the signature as a whole to appear right after the signature declaration.

(sigelem sig-id id)
Typesets the identifier id with a hyperlink to its definition as a member of the signature named by sig-id.