get-racket-referenced-identifiers only works on host, fix docs and errors

[michaelballantyne]

#lang racket

(require syntax-spec-dev)

(syntax-spec
  (nonterminal dsl-expr
    (dsl+ e:dsl-expr e2:dsl-expr)
    v:racket-expr)
  
  (host-interface/expression
    (dsl e:dsl-expr)
    (displayln (get-racket-referenced-identifiers [racket-var] #'e))
    #'#'e))

(let ([x 6])
  (dsl (dsl+ x 5)))

raises

dsl+: dsl-expr may not be used as an expression in: (dsl+ (#%host-expression x) (#%host-expression 5))

which is not a nice error.

Happens because the operation local-expands the syntax. Should add a check that it's a host-expression form first.

[michaelballantyne]

Or, traverse to find host-expression forms and then local-expand just those.